Sorry if I wasn't clear... These were not necessarily expected to solve the issue, but rather some general advice. If sae-mixed actually works in your situation, you can keep it as is... but many people end up having issues, so the fix is often to roll back to wpa2.
That is supremely annoying! It is unusual for a consumer router like this to use two IPs on the same network, so I wouldn't have guessed it directly. However, the general idea of another device inadvertently conflicting is why I had said this
A really quick and easy (but not 100% reliable test) is to do a ping test to the address with your device powered off... if you get a response, something else is clearly using the address you are trying to use. ARP tables and other discovery methods obviously also work, so that was great thinking. Despite the annoyance towards that ISP box, I'm glad you were able to find the problem... now you can use another address (maybe 253) and you should be good.
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