Just to clarify... Are you saying you can establish an internet connection, but even when a successful network connection is established, the VPN will not connect? Also, please ensure there are no other active connections on the PC (make sure it is not docked, no lan cable, no other WiFi connection established,etc)
Another question, is the device tethered with USB or are you connecting via WiFi? Test both - see if one connection method works and the other does not.
Finally - is this a prepaid hotspot plan or a monthly account? If prepaid, beware of webguard. It is added by default and is intended to block adult content (not that your VPN is adult content, but sometimes the shark net catches other fish by mistake if you know what I mean) - if you have this feature, get it removed.
If everything has been tried and you know it is not the VPN configuration and you are able to browse sites on the device, the last step I can think of (unless any of my comrades here want to chime in with other ideas) is to call Customer Care and have a trouble ticket opened to see if there is something on the network side preventing the connection.
Thanks for the thought you put into your reply!
1) Internet connectivity is OK - including access to the VPN gateway (i.e. we can ping it as well as initiate VPN handshaking).
2) there are no other active network connections in parallel.
3) local connection is wifi. USB does not help because we need to support several VPN session from several laptops at the same time (as I said in another post we are also trying to connect a printer to the hotspot for workgroup use).
4) we tried both with a data SIM card with a prepaid, pay-as-you-go account and with a data SIM card with a monthly contract (used in a single-user stick).
5) webguard was removed as soon as I saw that it was configured on the prepaid account. I also asked the T-Mobile technical customer support whether other"features" are in place such as checking for unregistered domains, etc. (he said no). I also tested the connection again 24 hours later just in case the webguard was not removed immediately but scheduled for an overnight maintenance job.
6) I did call T-Mobile technical support to check whether there is a clear channel to the internet (after removing webguard). They said it is a clear channel and didn't open a ticket.
7) I am also checking on our side if there is a problem with the IP address that presents itself at the gateway.
8) I am posting this from within our VPN using a Verizon MiFi that we can use temporarily while resolving the issues - which serves to localize the issues . . .