How to resolve OpenVPN not working on some carriers

I recently had an issue with OpenVPN not working tethered to my iPhone. This only affected my FreeBSD Laptop and not my MacBook. Curiously they are using the same configuration file. Note that while it didn't work tethered to my phone it was working when I was on other WiFi.

The issue was it was able to connect but, I could not browse any systems behind the VPN. In this case the VPN is for remote access to my home network. Not necessarily for browsing the Internet, though it is configured for that as well.

Note this VPN is going through a PFSense Firewall so you would think it would work out of the box on FreeBSD. But I guess that's IT for you.

Anyway after much agrivations including going down the path of thinking it was the firewall on my Laptop I eventually found the issue. All it took was adding the following to the openvpn file.

tun-mtu 1400
mssfix 1360