My internet is horrible.?

[Help]

Hey guys,
I've recently switched to iPrimus and my internet has been fine, but when i play PS4 i constantly lag..

My modem is downstairs and my PS4 setup is upstairs, i wanted to bring my modem upstairs so i can connect a ethernet cable into my ps4 from the modem.. But i cant do that cause the home phone is downstairs connected to the modem and for some reason, we cant take both upstairs.

How do i improve my internet on my PS4.. I have called iPrimus and there all logical answers that have never done anything.

Also whats the difference between a router and modem, can i buy a better router to improve my internet connection? If so, can you please link me one that will work with iPrimus.

My budget for a router would be $400, ty.

[Boss]

Change your ISP.