WiFi allows Apple to determine location precisely without depending on the router. Ethernet OTOH won’t play nice with location. Combine that with a China-made no-name router loaded with insecure, open-sourced software/firewalls, and you have a recipe for mystery problems that all point to the router.
Unless you replace your router with a known, branded router, it will continue IMO to be the prime suspect. Assuming you aren’t using a cable longer than 6’, replace the router. Or toss the Ethernet connection and stick with WiFi. I don’t see any other option (but hopefully I’m proven wrong). Good luck! 👋