The keys were RSA and generated via ssh-keygen -t rsa -b 4096
SOLUTION: I generated a key at probably a weaker strength but now it is accepted on all three routers.
ssh-keygen -t ecdsa
Got some older ASUS routers here, upgraded to Merlin firmware. This used to work for me, not sure what has changed but now I can't login to the older models with ssh keys. I can still login manually though.
I have rebooted them, re-did the keys that work on the RT-AC68U and applied them to the...