Free routers are better than they've ever been.
I currently have a mesh WiFi network with Huawei Mesh 3 access points, which are horrible. The can't daisy chain, so the satellite nodes must connect to the main node (which is connected to the router ...
The router your ISP gave you is almost certainly the weakest link in your home network. TP-Link's Archer AX21 fixes that for $51, at a record low with no Prime membership required.
If your internet is always sluggish, you can make it faster by boosting your Wi-Fi signal. Here's how.