Sounds like your trying to connect to AWS Client VPN to access your AWS VPC, and have a DNS issue, Title is abit misleading, AWS use OpenVPN, so you basically get the service open on UDP port 1194, and yes that works over the Mobile CGNAT network,
however you’ve possible UDP blocked, try bring up the connect using a different client or over the OpenVPN CLI, as you can download the configuration
Connect using an OpenVPN client application - AWS Client VPN (amazon.com)
also check if you have issue connecting to the VPN, and compare your public IP e.g https://ifconfig.co/, if you get a sunrise or amazon one,
resolve the VPC IP of the RDS endpoint before connecting the VPN, and see if you can access it. e.g 10.x.x.x or 172..x.x.x etc..
But this doesn’t look like a specific sunrise issue, as I use AWS Client VPN daily also on Sunrise network