You'd normally just search "[email protected]" from kbin.social to federate it with this instance but that doesnt seem to be working. There were a lot of lemmy instances with this issue but its becoming less and less as they get patched.
Once its federated to kbin.social your /m/ URL will work.
No problem. That is correct; there is nothing you can do. A temporary solution would be to make an account on a lemmy instance that they are federating properly with.
A week or two ago there were 10+ instances with this issue and it seems to be down to only a few now so hopefully this won't be an issue for much longer.