A repeater takes the wifi you have and blasts it louder so it can go farther. An access point is an entry point into the wired network. So, you could have wired as far as you can practically take it and then make the final hop wirelessly.

You’ll have to penetrate somewhere if you can’t go under the door. You might have to just have an access point at the end of your hallway and connect wirelessly from there.

Every connection takes 10 meters off the maximum run as a rule of thumb. If every cable is terminated perfectly, and every jack exactly to spec, and all the stars are aligned perfectly, it shouldn’t do anything.