Anytime you have major speed issues, 1) power modem, router, and PC off then back on. wait a bit and then test. 2) bypass the router by connecting your PC directly to the modem to see if it is your ISP or the modem. 3) If it is the router, then reset to factory specs. You will need to reenter your data but do it in steps (ie test connected PC before turning WIFI on)... Let us know what works for you.

already done that.

I assume the PC ran at full speed when connected to the modem. Be sure you have a good cable between the router and the modem and a good connection. Be sure you are not running any VPN as that will slow you down. Did the speed change when only the PC was connected to the router via ethernet when WIFI was off vs when it was on?