This is very odd.  I thought V3 and above had this option (my box says v4).  So I did some digging (https://www.tp-link.com/uk/support/faq/2719/) and it seems you need to set it to Wireless Router Mode, not 4g/3G router mode.  Ime a little confused.  Thought the whole point of the backup was to fall over to $G/3G if the upstream lan did not work (so you plug one of the 4j45 ports from the cable model into any of the rj45 ports on the back). 

Can someone please help me understand, my head is spinning ;).

Ben