Only functions fully over wifi, not 3G/4G

[s13487]

I have duplicated this issue on 2 servers on unrelated networks:

Server 1: Ubuntu 14.04, OpenVPN 2.3.2 (have just found 2.3.8 is available via the repo http://swupdate.openvpn.net/apt so I'll try updating to that)
Server 2: Asus router with Merlin Firmware, OpenVPN 2.3.6

Device: iPhone 6S, iOS 9.1, OpenVPN app 1.0.5 build 177 (it is not jailbroken)

I have turned up the debugging on the iOS app, no errors shown. Checked server log, no errors either.

Both VPN servers exhibit the same problem. Asus router OpenVPN server was set up via it's web GUI and the Ubuntu box I configured myself from some blog post guidance. So it isn't a case of one config copied to another.

I have seen someone else has this problem (they replied to the IPv6 dual stack thread yesterday, so relatively new problem?).

On wifi it all works fine. On 3G/4G strangeness occurs.

Does not work:
- Can not ping anything (tried 2 apps, Fing and Ping Lite), the OpenVPN server or anything on the LAN
- Microsoft RDP app will not connect to any Windows servers

Does work:
- Can access any servers via HTTP or HTTPS in Safari OK
- Serverauditor can SSH to Linux boxes fine

Examples as this is odd/confusing!:

- Windows box: can access HTTP/HTTPS fine. Cannot ping or RDP it.
- Linux box. Can access HTTP/HTTPS and SSH fine. Cannot ping it.

Sounds like certain apps aren't accessing/seeing the TUN interface or something when 3G/4G is active, only on wifi? (This is a rough guess with my small amount of understanding/knowledge)