Leaving this here in case anyone else has the same issue. I was incorrectly scrolling to the bottom of the models list and choosing T46 when I should have picked T46U.
This usually happens when the old router phone hasn’t fully cleared from the system even though it was deleted. In 3CX, router phones can sometimes “stick” in the provisioning state, so the UI ends up thinking there’s still an active device linked to that extension. That’s why all the options go grey. sometimes this helps normally:
restart the 3CX system once
check the extension and make sure nothing is still attached there
remove any leftover provisioning entries if they’re still showing
then try adding the router phone again
Most of the time it’s not a real config issue, just a stuck state after deletion.