I have actually already tried this. The phone was set to use a random MAC address, so I turned this off, leaving only the phone’s device MAC, which is one of the two you mentioned, in use. I then went into the router and set an IP reservation for the device MAC, using an address outside of the DHCP range on the router. (Is this what you mean by a DHCP lease? Or is that something else?)
Incidentally, do you have any idea why your phone has two MAC addresses - Smartphones usually only have one as most smartphones only have a WiFi NIC.
The DHCP "Scope" is the range of IP Addresses being serviced by your DHCP Server. "Scope" is (I think) the terminology Microsoft use in their DHCP Server, I'm not sure if that's reflected on other platforms. But no matter, I refer to the range of IP addresses you DHCP server is managing. You can usually specify it, even in basic SOHO kit you can often set the start and end address of the range.
A DHCP "Lease" is the name for the thing that the DHCP server gives out which, amongst potentially hundreds of options, often includes the IP Address, lease length (and in SOHO kit) the default gateway and DNS Server addresses which are often your router's LAN IP address.
Some DHCP Servers won't offer out a DHCP Lease (reservation) if it's outside the Scope to you might perhaps try revisiting this and ensure that any DHCP Reservations are within the available scope (and aren't in use by anything else of course.)
Be aware that whenever you set up a DHCP Reservation, it may not come into use straight away. A client that already has another DHCP Lease "in hand" is entitled to keep using it until that lease expires. Only then is the client required give up a lease and solicit a new one. Though, IIRC
@oneman mentions a few post back, DHCP Clients normally start requesting a renewal of their exiting lease at 50% of the lease time. Point being, having set up a Reservation, it could be some time before a client device claims it.
Of course, that also points to the fact, if it's not already obvious, that client devices "pull" their DHCP Leases from a DHCP Server, the DHCP Server does not "push" them to the client. Basically it's a four step process -
1) the client says "can I have an IP address" (Discover)
2) any DHCP Servers that hear the "Discover" will send out an offer of a lease (Offer) (if they are so minded.)
3) the client then declares which Offer it wants to accept (Request)
4) the DHCP Server that made the Requested Offer will acknowledge the Request (and any other DHCP Server will then withdraw their Offers.)
Renewal of a lease in hand is similar, but it more or less skips the first two steps. Wiki's article on DHCP is quite good if you really want to get into the weeds.
Either way, this didn’t stop the conflict - every time the phone connects the same conflict message appears in the router log. But it doesn’t stop the phone connecting, and the phone ends up with the IP address I have allocated to it and can happily browse the internet.
The only thing I didn’t do was tell the only E to forget the network, power cycle and connect again - that can be tonight’s activity.
Final thought now is that I’m still not sure that the drop outs in connections are anything to do with this conflict - half the time we don’t get a drop out and need a router reboot when the phone connects, at other times we do. It just seemed a bit of a coincidence that there’s a conflict with the router’s own IP address and our internet connection is unstable for other reasons (eg Three being rubbish).
This is why duplicate IP addresses using your routers IP Address are particularly troublesome. If "something else" starts using your routers (LAN) IP addresses (whether it obtained the IP address through DHCP or any other method) then it effectively knocks out Internet access for everything. It "looks like" the Internet link has gone down, because no-one can get on the Internet, but the link is probably fine and just that no traffic can reach it. And (usually) you won't be able to log in to your router to see what's going on, so you might also think your router has broken, crashed or locked up.
Imagine it like the postal service. Normally, when I and everyone else in my street put our mail in the postbox, a postie comes along and takes it to the local sorting office and off it goes wherever and of course, replies to our correspondence come back the other way. But now imagine something goes wrong and instead of the posties collecting from the postbox, someone else does and takes all the mail off to (lets say) a land fill. Our mail never reaches the sorting office, consequently never gets forwarded anywhere else and of course that means we get no replies. To us, it looks like the sorting office has disappeared, or gone on strike or something. It hasn't - it's still there and working just fine, it just looks broken because our mail is no longer reaching it and our correspondence is not being answerred.
It's pretty much the same in an IP LAN. Every IP device knows something called the "Default Gateway" IP address. Any device that wants to send an IP datagram to a device not connected to "the same" network as itself (it figures this out from it's IP Address and Subnet Mask) sends it to the the Default Gateway and that device then forwards it elsewhere. In a SOHO LAN, the Default Gateway is your router. Thusly, if anything hijacks the router's IP address (deliberately or accidentally) then all the traffic that should be getting sent to your Default Gateway (router) for onward transmission to the Internet gets send to the "bandit" device instead. And who know what happens to it once it's there - it probably just disappears. And, just like my postal metaphor, because no traffic reaches your router, your router isn't sending it to the Internet, and it's not getting any replies. So it looks like "the Internet has gone down." And to rub salt into the wound, the same IP address is also used for your routers admin interface, so you loose connection to that too which looks like the router has "broken."