VPN not allowed with my T-Mobile router

[gvglaze]

Hi - i have a question and hope to resolve.  My son has left me this cool t-mobile router that is stand alone, pulling from the nearby celltower, i think.  It looks like a fat alexa. (NOK 5G21 Gateway)
Anyway, I get really good internet, but only if I turn off my AVAST VPN.
I would like to surf with the VPN on.  I have another source of internet, much slower, and allows the VPN to work.
Can anyone assist?
George

[Asyn]

-> https://support.avast.com/en-ww/article/222/ (Changing DNS settings)