Hi, jacob2!
Have a good day!
Please check the configuration first.
If the SC is not configured with a NAT address, extranet terminals will fail to register with the SC.
You can check the address on the SMC, see the picture below:

If the configuration is correct, check the network.
You can capture packets to check whether the network is faulty.
If no registration message is received, the network is faulty. Check whether the network NAT is normal and whether the port (TCP 5061 or UDP 5060) is enabled.
Best wishes!