I have Gigabit fiber with a Sonic supplied SMART/RG SR515ac Router.
This works fine except that using any browser in macOS Big Sur I cannot get the router's main admin page to display properly. In particular the control panel to the left of the page is always just a solid green - no menus items. See attached.
When I use an older version of a macOS, or a browser on a mobile device, or a browser on Win10, I don't see this behavior.
I've tried all the possible variations of Big Sur's security settings that I can think, but to no avail.