Let me explain my home set up first

I have a 300 mbs fibre plan and the router is in the garage because that is where the patch panel is.
Everything that is connected via ethernet, works fine.
Wifi becomes marginal at the farthest point from the router.
I have multiple ethernet ports around the house, which includes one port in the study (farthest point) and one port in the bedroom (next to the study).

In the bedroom I have a new Panasonic TV (TH-43LX800Z) and when connected by ethernet, I have no problems.
However the bedroom ethernet port has now been used as an HDMI Extender (and that works perfectly), which means that I have to use WiFi for the bedroom TV.

I installed an Edimax AC1200 Dual-Band Wi-Fi Access Point in the study which is wired directly to the router using the study ethernet port.
I connected the study computer to the AP by an ethernet cable and, using the Ookla speed test, I get 283 Mbps download and 97 Mbps upload.

The AP is set to broadcast on the 2.4 Ghz and 5 Ghz channels and using a Samsung S9 phone in the study, I get 265 Mbps on the 5 Ghz band. I get 232 Mbps in the bedroom standing next to the TV.

I have set the TV to connect to the 5 GHz band and this is when I have problems.
If I am watching live TV, a message continually flashes to say that the WiFi is not connected.
If I try to watch an App (say TVNZ+) I have trouble connecting and if it does start playing, I get the attached message:

Note that although the picture has been replaced by this screen, the sound track keeps playing.
When I click on "try again" the picture comes back, but goes again shortly after.

I have tried the 2.4 Ghz band, as well as the Router Wifi, with the same result.
I have also tried watching on an iPad and it works fine.

Any ideas? Please don't suggest running extra ethernet cables, as that is not an option.

Thanks