For the BT router, you will need to open up the ports required on the router (port 80?) if you are using NAT mode (which by description you appear to be doing) - see your router manual for how to do that.
Then, as you need access to internal devices on your network and it's got an MT in between, you need to look at dst nat masquerading. Incoming packets of data, hitting your firewall on the public internet side, will have a destination IP address which is the public IP you have been given by BT.
But you need those packets to be routed through to a destination IP of your IP camera which is on the internal network. That is why the name for this is dst-nat. Destination Network Address Translation.
So, presuming that the IP camera only needs an incoming connection on port 80, as if it were a web server, try this,
/ip firewall nat add action=dst-nat chain=dstnat comment="Incoming feed to IP Camera" disabled=no dst-address={external IP} dst-port=80 protocol=tcp to-addresses={internal IP} to-ports=80
Change {external IP} and {internal IP} to suit. If it is not on port 80, but port 443, change accordingly. If you need to dst-nat other ports becuase the camera requires other ports to be opened, just add more of the above rules, changing each one as required.
With the router doing NAT and the MT doing NAT, that is called double NAT'ing, which can give problems. Try if you can to place the router into half bridging mode. This means the router will do the PPoA authentication, but pass all packets destined for your single public IP straight through the router untouched to your MT external interface.
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with a 2wire Bt router.
