Question about Teltonika TRB140

When I ordered this product, a SIM card was offered free of charge. I can surf without any problems, but I cannot set up a VPN tunnel. I suspect that this is due to the subscription because I get a WAN address in the 10.xxx.xxx.xxx/32 network (using WhatsismyIPAddress I see a public IP). Trying with a second SIM has given the same result. I have read that such VPN connections with so-called "shared IP" addresses do not work. Does anyone know if it would be possible to get a subscription that assigns a dynamic public address and then work with DDNS to create a VPN tunnel? (We are talking about private trials here. I don't want to go the way with a fixed public IP). Thanks for any suggestions!