Connecting complete wifi disc to an access point via Ethernet cable

[GG76]

I did this with my old white BT discs, connected one of the discs in the house to an ethernet cable which is run out to a garden room about 15m away, with a TP link wireless access point on the other end of the cable in the garden room. 
I have tried this with my new black complete wifi disc but it doesn’t seem to work and the blue light on the disc changed to red with the cable connected to it. Any ideas if it is possible to get this working?

[GG76]

I’ve just tried putting another black disc in the garden room connected to another disc in the house by Ethernet cable , rather than using the TP link, it works but means I need to take a disc out of the house set up. Would be handy if I could use the TP link in the garden room instead.

[RJeremyB]

I do not wish to hi-jack the OP’s thread, but I have my two penn’orth and a question for the gurus.

I live in a 1900s house with the associated WiFi problems, which I have addressed with a Smart Hub2 and number of BT Complete WiFi (black) discs.  Where possible, I have connected the discs back to the Smart Hub using ethernet.  The other discs negotiate a connection with the rest of the network.  It works and when it works I have excellent speed anywhere in the house.  For the OP, I would opine that if his system works, crack on.

However, I am chasing a problem wherein occasionally devices report that our WiFi network would connect, but not give an internet connection.  I have spoken to a number of BT/EE guides and last week one told me that there was “WiFi congestion” and that one of the settings in the SH2 was being altered.  Regrettably, I forgot to ask which setting, but I guess that it was to move the channel setting for either the radios.  It worked, until today. 

Today: same problem.  I spoke to another of the BT/EE Guides, who said that if a disc was connected to the SH2 by any form of ethernet (switches, Powerline, direct cable), the disc would not form part of a mesh but would become a single WiFi node and that this was causing the “WiFi congestion”.  I had previously understood that connecting via ethernet was advantageous, giving a high-speed back-haul for the relevant disc.

Can anyone confirm or refute the advice that I have been given?