So your WiFi at home isn't great?
First, make sure that you're getting a good connection when you're near the router. The easiest way is to get close to it with a computer or phone you find to be "fast enough" and go to https://fast.com in a web browser. You'll want to make sure the result you get is reasonably close (within 10-15 percent) to the promised speed of your connection from Comcast or whomever. That will make sure you don't have to replace the router anyway – you might expect a router to last 5-6 years, give or take. It'll still work after that, but it probably won't keep up with the speed increases of your internet connection.
If you aren't getting the speed you expect, you should probably get a new router. That may not solve your coverage problem, but if you're not happy with speeds close to the router, you won't be happy when you're far away from it.
Okay, so once you're happy with your performance near the router (or know you need a new one), there are a couple