Have you also tried the ff: turning the modem and any router off and on, trying another device, like a phone and laptop, using the same connection (wifi and/or wired), probably safe mode with networking, clean uninstall all VPN networks, disabling and enabling the network adapter, uninstalling and reinstalling the network driver, etc?
I remember not being able to remain logged in in one forum: it would log me out after I visited a few pages. The admin said that it might have something to do with Cloudflare, etc., and using another browser, etc., clearing the cache, clearing cookies, using a VPN, etc., using a laptop, restaring the modem and router, etc., didn't help. It continued even after I did a repair upgrade of the system (Win 10) given another problem.
Later, I closed my fiber account and upgraded my landline (which I had for a long time and needed for work) to use a new fiber account with the same ISP, and the login problem disappeared.