I have a sricam camera which I can access from outside but I connect to it via sricam's servers so presumably its managed in some way rather than just connecting via IP address and port.
Both DynDNS and No-IP work (currently) with my Three HomeFi MBB set-up and their supplied MBB 24 month contract SIM.
I haven't tested it with the Three 'free 200mB a month' PAYG SIM or a 'phone' SIM.
CG-NAT on Three MBB has been discussed before, it seems that making sure the APN for the Three MBB service/router is '3internet' for the Three MBB sims is the key. The Huawei B311 defaults to this.
Remote access into the LAN from the WAN side using the No-IP free service works fine for FTP servers on my NAS - but the No-IP free service needs a monthly renewal to keep it enabled. They send a reminder email every month so it's not a major hassle. Useful for testing.
Ports also need to be opened at the router to allow WAN connections through, which is either a major security leak, or a useful convenience.
The B311 Homefi does support DDNS for DynDNS, No-Ip and Oray connections.
As always, this info might be out of date, so it's wise to double-check before signing up to a contract.