Depends on what you are trying to do. I can see where having two WIFI close together may cause an issue if both are on the same channel. You may want to change the Advance WIFI settings of both WIFIs...

Check to see if your ISP address was changed by your ISP.

If you bypass the router and connect direct to the modem via wire, what speeds do you get? Also try different cables as sometimes they can affect speed.

If it is not working, use your warranty. Do not open it as that may make your warranty invalid. Try the 30/30/30 reset...

You are correct. Limit would be 100. https://www.tp-link.com/la/home-networking/wifi-router/archer-c60/#specifications

1) https://www.tp-link.com/bg/support/download/archer-c5-v4/ assuming region and language. 2) TP-Link changed the user interface (UI) one is green and the other is Blue (older). They are different so...