RF wiring issue maybe? You don't say if the sets receive terrestrial TV (Freeview) from an aerial and the same distribution system...
First : Move the TVs to the known working outlet and tune them there. If they work then return to the correct room and sort out why the RF feed isn't working there (remake plugs sockets, move hdmi and aerial cables apart etc.,.)
If modulator is still not found:
The Triax modulator is DVB-T H264 (HD) codec.
Old TV may be DVB-T only and not Freeview HD which mandates DVB-T2, EU sets of a certain age would include the H264 codec so it will work OK = picture and sound.
Ensure the newer DVB-T2 sets are manual tuning set to DVB-T not -T2? (Perhaps?)
Other things to try will be removing the D-A (in case it's over-amplifying) and direct-connecting each TV to prove that... Either passive splitting or a lower gain amplifier?