New routers won't always fix a "weak WiFi" problem. It could be that the construction &/or layout of your house is part of the issue. In older houses, wire lath can block the signal.

In houses of any age the signal can be blocked by appliances or solid wood columns and walls.

There are apps for smartphones that will show you the WiFi signal strength as you move around your house. Use one of those apps to map the signal in your house and find out where the weak spots are. If it's weak everywhere, then maybe start with a new router. But if it's weak only in spots -- especially just after you've put in a new router -- then you need to look into adding one or more WiFi extension access points, or perhaps installing a WiFi "mesh" system, such as "eero" or "Google WiFi".