We've had a BT Smart Hub for around two years, supporting several devices (tablets, laptops, Firesticks etc.) primarily connected by WiFi. Most used 2.4GHz (split SSID) because it was more reliable around the house. There haven't been any noticeable issues until the past three weeks or so, with drops in reliability and speed frequently occurring.
Running browser-based speed tests (Speedtest.net, Google's speed test and BT Wholesale's test) on 2.4GHz and 5GHz connections, there's frequent variation in results: downstream (Mbps) can be in the low single digits and sometimes up in the 50s, where I'd expect it to be normally. I can be stood within a few feet of the router (or the other side of the room) and still only get downstream speeds in the 10s or low 20s. There doesn't appear to be any particular pattern to it and I'm at a loss as to what the cause may be or how to definitively resolve it.
I've run a speed test with a wired device and it's exactly as I'd expect - low 70s downstream - and rules out broadband issues.
I've surveyed WiFi signals around the house with NetSpot (macOS and Android devices) and whilst there's a nearby router on channel 11, its signal is always weak (0-11%) and noise negligible (0%). I can't find any other devices on channel 36. Right now, in a neighbouring room with the door open (around 15 feet from the router), 5GHz measures at 37% signal/-61dB and -96dB noise. 2.4GHz measures at 43% signal/-60dB and -99dB noise. If I'm next to the router, the signals are -62%/-38dB and -96dB noise, and 65%/-35dB and -92dB noise, respectively. Whilst the values can vary a few numbers either way, the signal strengths seem low for the distance and conditions described, don't they?
On the router itself, Smart Setup is off, SSID bands are split and I've recently tried manually assigning channel 11 (there is no channel 13 to try).
Any ideas why this is happening now and what might be the cause? My best guess is the Smart Hub's had a firmware update or something environmental affecting the signal. The only thing I can think of to resolve it is ditch its WiFi and to get a mesh kit instead, but that's just a 'hope for the best' solution rather than anything I'm certain about.