ERR_HTTP2_PROTOCOL_ERROR This Site Can't Be Reached

Since last week, I can't access certain websites that I've used in the past. Such as, Costco.com, Marshalls.com, adidas.com..etc. I tried all recommended troubleshooting methods for "ERR_HTTP2_PROTOCOL_ERROR" that I could find online, none of them worked. I tried accessing these websites with the same computer through my phone's AT&T hotspot, and they worked fine. xfinity assistant's not helpful. I have reported these websites to xfinity. But I find new blocked websites everyday (so far mostly retail websites), so I am hoping to find a solution to this and fix it myself. Any help and recommendation would be appreciated. Thanks!