You really need to stop trusting what people say and giving in to buying replacement parts, systems to solve it. That makes you look gullible and some ISPs will take advantage of that by selling you things you don't need and over billing on monthly rates.
Also, any modem should have at least one WAN port just to connect your PC, so try connecting just it to the PC to see if your router is causing this.
Just so you know, for some time now many brands/models of cable modems have had built in routers, there's no need anymore to buy a separate modem and router. They also have all the benefits on speed and security separate modem, router setups have.
I have 100Mb cable service capped at 400GB per month, which is plenty for me, but granted I only play certain types of games. I went with the Arris SurfBoard SBG6580-2. With over 24,000 Amazon reviews and a 4/5 star rating, it's easily their most purchased, best reviewed cable modem, and comes with a built in 4 port router.
https://www.amazon.com/ARRIS-SURFboard-SBG6580-2-DOCSIS-2-4Ghz/dp/B0100KRBA6
It consistently tests at 106Mb on my 100Mb service, which is actually 1Mb better speed than the 1 gigabit rated modem/router my ISP first set me up with. It also has a 2 year warranty. These used to be made by Motorola, until Arris bought Motorola's modem division.
If you bought your modem and router at a place like Amazon and still have the 30 day satisfaction guaranteed return option, I would advise refunding them and getting the one I mentioned. It is never more than mildly warm and is a very solid modem/router combo.