Since my original post I replaced the two separate aerials (to get the 4g service) with a single 4g aerial, also from Connex Technologies. https://www.connextech.co.uk/lmo7270-wb-smsm---4g3ggsm-mimo-outdoor-omni-antenna-1153-p.asp
I specified 10m cables (as per the original aerials I fitted) and found it works superbly, even in the remotest of locations.
I don't think 5g will be so widespread in the near future, certainly in remote canal locations, to warrent changing the setup for a year or so yet.
Regarding 5g routers, many routers are labelled 5g when I understand they refer to the frequency, I think 5200mhz, not to 5g, the 5th generatation of the mobile network.
I am not so sure fitting the router itself atop a mast would be a good idea for several reasons. I would think that the routers internal aerial is not as sophisticated as the purpose made ones. I could be wrong on that though. And risk of water ingress and physical damage due to its vulnerable location when continually raising and lowering the mast.
Ken