Xfinity modem gets full download speed; TLINK and Netgear does not

A few days ago, my internet download speeds dropped from 200 MB+ to around 4.  I used the chat system with a rep and then talked to another with no help.  Their diagnosis was a wiring problem - they would send a tech out after the COVID restrictions lifted (no estimated date).  I asked them to send me their Xfinity XFI modem for the heck of it... thinking my old TLINK modem may be the issue.  Their modem arrived in less than 24 hours, impressive since they quoted 3 business days.  Much to my surprise, hooking it up - I got back to the 200 MB download.  So I went to Office Depot and bought a Netgear CM400.... not wanting to pay the $15/month fee to Xfinity for their modem.  I only need a modem - I have a wireless rounter with mesh system that I don't want to replace.  I hooked up the new Netgear router - and I'm back to 4 MB.  So, I switched back to Xfinity - back to 200.  Any thoughts on what is going on here?  By the way, my upload speed is consistent 10 MB - across all 3 modems.