Wondering if anyone can assist with an issue that I've encountered.

Scenario 1:
Modem / Router: Archer VR1600v (WiFi active – Pushing out signal on 5ghz & 2.4ghz bandwiths).
Internet: NBN 50 – Speed Test confirms DL 48Mbps / UL 12Mbps / Ping 9ms

Everything works fine in close proximity to the modem, but no service beyond 10-15 metres from device. The device lives next to my TV and so HD streaming is perfect but can't get WiFi upstairs on any device. Therefore, I've bought a new router to improve WiFi coverage. The router is TPLink / Archer A8.

Now..

Scenario 2:
Modem: Archer VR1600v (WiFi disabled).
Router: TP Link Archer A8 (connected to modem via Ethernet cable, sending out WiFi signal on 5ghz & 2.4ghz bandwiths)

Internet: NBN 50 – Speed Test confirms DL 50Mbps / UL 12Mbps / Ping 9ms

Everything works fine throughout the house until I want to stream live TV. I can connect to the service but encounter lots of buffering and stuttering. For some reason, the A8 seems to be throttling the WiFi signal to my TV / Firestick / Chromecast devices and I cannot work out why.

Have tried various setups including connecting the A8 to the TV via Ethernet, turning off 2.4ghz so only using 5ghz and vice -versa, but nothing seems to resolve the issue.

Does anyone have any ideas why the A8 is struggling when I want to watch streaming services but is fine for everything else ?

Any pointers would be hugely appreciated.