Hi Julius,
It is not possible to add a router phone if 3CX cannot register the MAC via the 3CX RPS.
The router phone setup relies on this to auto-provision itself.
If you previously registered them in the Yealink MCS you will have to remove them first otherwise it will be reserved and we cannot add it to our RPS entries. If removing them from your YMCS is not an option you will have to install a dedicated SBC.
STUN is no longer recommended due to the multiple issues it causes for many customers, and the extra support overhead needed, try to avoid it.