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tb-
Hadn't considered the Powerline option, would something like this be a good option?
Yes, they can work quite well as your aren't sharing the WiFi spectrum for the link between units. I use something similar, also Devolo, to get wired Ethernet to where I can't get a cable. Apparently I get 538Mbps between the units.
jellybaby
If you can get a cable between D and A then put a pair of APs with cables back to the router in D and A. Something like a pair of Ubiquiti UNIFI APs.
If you can't get a cable in you could go with PowerLine and make the radio hams sad but that works less well across different ring mains and there is a reasonable chance that you have a ring for upstairs and a different one for downstairs.
3rd option is as you say something like a pair of TP Link Deco. Again turn off WiFi in your router and then put one Deco next to it in D. Put the second in B so it has a decent signal to the other and likely a decent signal to A as well.