VoiP-Softphone doesn't work with Huawei Router which you get from Mauritius Telecom

I was so happy to get fiber glass at my place. But unfortunately I can't do voice calls with a softphone now as I could before when I want to call my clients abroad.

They can hear me if my softphone uses the UDP-protocol, but I can't hear them. Another softphone uses the TCP-protocol. That works, but the quality is poor.

After they changed the Huawei router against another one the technicians from Mauritius Telecom told me this router is the only one you get for optical network in Mauritius. But everyone I asked the last days told me it`s a problem with the router. What can I do? Seems there is no solution for my problem. But I can't believe.

Do you have an idea that can help me?

Many thanks for your help.