Troubleshooting something weird for a client...

Two temporary work sites are using a TP-Link Archer MR200 4G Router with a 2Degrees SIM card.  One laptop is normally connected via cable, plus a networked printer/scanner.

Client starts a VPN connection from within Windows 7 using the baked in WIndows VPN software to a Cyberoam VPN Firewall at the Head Office end.  At this point, for troubleshooting reasons, we start a constant ping to the internal web server (which runs a custom web application that has no security - hence the VPN being required to access it instead of it being available directly across the internet).  The ping responds fine.

If you then try to access the web application via FF or IE, the traffic across the VPN stops.  The ping stops, and the browser times out when trying to access the internal web server over the VPN.  Internet access is fine (as the 'use default gateway on remote network' is unticked ion the VPN properties).  Disconnect the VPN and try again...  same things happens.  Disconnect the VPN again, use Chrome, and it all works fine.  The ping keeps going, and the application works fine.

If the guy tethers the laptop to his iPhone, the VPN works fine and he can use any browser.

A case has been logged with TP-Link, but to be honest I'm not expecting much of a response.

Any ideas or suggestions would be appreciated.