Does the BT Mobile network support outgoing L2TP connection to the other VPN hosts? As I tried to use L2TP connection with my mobile phone to one of the L2TP VPN sites, it is not work. When I try to connect it after connecting with the wifi network, it works fine, so I'm sure my L2TP VPN setting should be correct.
It's probably a combination of L2TP and CG-NAT (Carrier Grage Network Address Translation). It's not that the combination wont work, it's more that depending on the client it sometimes won't work.
And the fix is?
Is there any way you could try using WireGuard instead? It tends to work better with CG-NAT! Sorry, that's the best I've got on this!