Problem with VoIP phone and MTN LTE Router setup

Good day, I am having a poor experience with the implementation of a VoIP phone which I am installing for a client of mine. His client code is WAR153. This client has an Fixed LTE service through Axxess as well. Please see the below the Sequence of events: 1) The Client app**** on the 8th January for the yealink phone but AXXESS only processed their application on 16th January and then received the IP phone 2 weeks ago. 2) Upon Installation, I entered the correct SIP server details, username and password but the IP phone failed to register. I contacted Axxess support and was advised that the public IP needs to be unblocked by the end of the day 3) Second day, the IP phone still failed to register and was advised that the public IP wasn't unblocked and was to be unblocked by the end of the day again 4) Third day, still failing to register, I contact Support whom advised that public IP was unblocked and that I needed to test the account on the Zoiper App. I downloaded the Zoiper app, scanned the client's QR code. The Zoiper App failed to register when using the Clients's WiFI network (which is using the LTE setup mentioned above) but when switching to my data connection it worked. I contacted support again via email and 2 days later I was asked for the Client's contact number I omitted it by mistake.

To date, my client has paid Axxess in full for a service he can't use and Axxess has provided me with sub par level of support with no resolution in sight. Based on the above steps, the issue is clear that the problem lies with the LTE router. I have checked the LTE router and SIP ALG is enabled. The client has tried turning it off for 30 min and tried testing his IP phone and it still isn't working.

I have referred Axxess and signed up quite a few clients (and can give their email addresses if requested) and feel that I can't trust Axxess to deliver a proper service based on this experience.