(I'll give you guys a good hint/suggestion)
If you're like me, the reason why you have to use 4G is because the cams are going to be in a really remote area and likely several hours away. Plan for multiple services, not just one. Relying solely on OpenVPN or any VPN service you choose as your primary is a bad idea. Allow yourself some troubleshooting flexibility. A backup dial-in service running at any layer, even if it's not the quickest or best, is ideal. Also add an IoT device or two that doesn't rely on VPN to communicate--something to test your connectivity outside so you know what to pack if you see the system is offline. IP Phone, WeMo thing, RPi, etc--just something that gives you that little extra knowledge of what point the system is messed up.
Alternatively, or additionally, skip all of the consumer crap routers and modems. Stay away from hotspot pucks and anything involving a battery that is plugged in 24/7.
Do some research into DIGI International before you do anything with this project. They support OpenVPN, IPSec, etc and have their own remote management capabilities where you can fire off code to or get statistics from the cloud for the exact reasons I mentioned above. They also run at like 150F and -40F without losing a packet and can be run over by fully packed X5 (from experience).
Dual SIM and dual WAN capable. Again, can't go wrong.