No ethernet connectivity from RBR50 (backhaul or devices)

I've fixed it.

I would never have known this was even a 'thing', but the unmanaged switch in my lounge - between the router and the switch into my dining room - was the culprit.

I narrowed it down to the switch, so I unplugged it and tried it elsewhere in the chain and it worked without issue, which made me doubt whether that was the issue after all. I then positioned back in its original spot, plugged it in and all of a sudden, it's working without issue. I systematically plugged everything back into it, and everything is now purring like a kitten.

I did not realise that simply power cycling a dumb switch could fix the problem (given that there are no 'brains' going on in there) nor would I have thought that that could cause the issue in the first place... how has it managed to cause an issue in the network when all it's doing it providing more ports in the network?

In answer to your latest question though, FURRYe38, so as not to be rude when you've been helping me out...

The switch in question is a Netgear GS316-100UKS 16-port Gigabit unmanaged switch. Cables are mainly Cat.5e given that Cat.6 were too thick to manoeuvre around where required (i.e. when not in a straight run). My broadband is only 45 Mbps so I'm not suffering by using Cat.5e, although ultimately I would like to move to Cat.6 as a minimum, but that's a MAJOR job involving running cables through walls (I live in a brick house) and around bends and doorways.