I have everything working as prescribed in sajunky's post, I now get the internet from Router B - but I'm still having issues with port-forwarding, I don't know what I'm doing wrong.
I think the settings above are correct as I'm getting internet on all devices connected to Router B
Yes. Your setup looks fine to me, and you have internet on segment 192.168.2.x.
Could it be the firmware version?
No, that's the latest firmware for the B593-s601. No problem there.
I'm worried about the line that reads "Remote authorized access will override the inbound policy." on the firewall screen above?
That line just means that any port opened explicitly will be forwarded, irrespective of the inbound firewall policy. No problem there either.
If you're running the B593 in DMZ mode, ALL inbound traffic on all ports is being forwarded to the DMZ host (192.168.1.2), to be handled by the Zyxel router. This is irrespective of firewall and port-forwarding settings on the B593.
I tried removing the port forwarding on B593 but still I couldn't access that port from outside
As mentioned above, the port-forwarding rules on the B593 should have no effect for traffic forwarded to the DMZ host.
the DVR is connected to Router B on IP 192.168.2.5, I can access the cameras if I use that IP on a local client. The port forwarding setup above used to work fine on this router when I had ADSL. I don't know what else I could be missing
I do hear what you're saying about the setup having worked on ADSL before, but are you sure that your DVR needs only 1 port to operate ?
For instance, on the setup for my cameras shown
here, all four ports are needed for the cameras for negotiation, config, streaming etc.
Obviously when you access the DVR from the LAN, all ports are implicitly open. When coming from the WAN side only the ports specifically allowed by Router B (currently only port 37777) will be open. Double check your DVR documentation to see what ports are needed and check that you've opened them all.
You could also try disabling the Zyxel firewall and opening all ports on the Zyxel (temporarily!!) to see if you can get remote camera access that way.
If none of that works, I'd suggest temporarily connecting the camera direct to the B593 as the only client, and placing that client (the camera) in the B593's DMZ. The easiest way to try this is probably to keep the B593 setup as it is (LAN IP at 192.168.1.1; DMZ host at 192.168.1.2) and re-configure the camera to be at 192.168.1.2. That way the camera is connected as directly as possible to the internet. Then see if you can get access from outside (like with a laptop on 3G) and if it works, work backwards from there.
Lastly, it could be an issue with the LTE APN having some port restrictions, someone else may know more about that.