Maybe it's worth visiting a few fundamentals of how Wi-Fi works which may thence help with understanding and hopefully inform decision making:
One of the biggest Wi-Fi myths is that there is something called "Wi-Fi signal" as if it's some ethereal energy field like The Force or Ley Lines or something. There is no such thing as "Wi-Fi Signal" - it doesn't even work as a metaphor.
Wi-Fi works like sound, only it used radio transmissions instead of audio transmissions. So ask the question "if I was "fixing" my problem if it was using sounds, how would I do it...?"
Wi-Fi is a two-way radio "conversation" between communicating peers like walkie-talkies, not a one-way "lecture" like television. Every Wi-Fi device on the planet - phones, tablets, laptops, printers, HomePlugs, AP's, routers and everything/anything else are all radio receivers AND transmitters. Only one thing at a time can transmit - I talk you listen, you talk I listen. The more "things" there are, the more they wish to say, the more competition (it's anything but "fair") there is for some "air time."
Wi-Fi is deliberately "quiet" and the transmit power is limited by law. Pretty much everything is, and always has been, transmitting as loudly as legally possible. There are no "magic" anything with "much better signal" than anything else. Though there are a few "tricks" to make it appear so.
At a basic level, since everything is already being as "loud" as allowed, if client-AP pairs cannot "hear" each other very well, then we either have to move them closer together, remove the structures between them or a combination of both. In most homes, I suggest taking down the walls is impractical so we are left with getting the communicating peers closer to each other.
Thusly, unless we are going to move all our clients next to our router/AP, then the remaining option is to deploy AP's nearer to where our clients are creating a "cellular" pattern of hotspots. Usually that means adding additional AP's closer to where we do most of our Wi-Fi'ing (if we cannot move our single "router.")
Things like "mesh" systems, Repeaters, (also sometimes given names like "Extenders,") or just stand alone AP's achieves this. The "trick" with such things is how we establish the backhaul links between them as discussed previously.
And finally a clarification of nomenclature: All Wi-Fi is availed by Access Points (AP's) not "routers." This is not just hair splitting over nomenclature, in the field of data networking an AP and a router are very different things. The "get-you-on-the-Internet" omni-box we have at home contains both and much more besides. AP's get built into many things such as SOHO "routers," Repeaters, Extenders, Mesh nodes, (some) HomePlugs and so on. To deploy additional Wi-Fi hotspots and build up a "cellular" coverage pattern, you only need additional AP's not "routers." "Mesh" systems are AP's with some extra functionality and often a common/integrated management platform.
I hope that's useful.