Router resetting and speed

[#1]

I have recently upgraded to a faster Plusnet service. However often when I check on the speed, it is below that of my previous service speed around 22-25 d/l.
I reset the router and I after this completes I get speeds of around 63-65 d/l. However perhaps a day later is has 'reset' to the slower speed. I've checked the connection is to the 5Ghz bandwidth each time, although I'm not sure this would impact the speed so dramatically anyway.
In addition to this issues, while in use the router resets itself (not sure if this is WiFi or complete reset) this leads to outages. Unfortunately I run a business using video conferencing so it's meant cut offs mid calls. Any ideas what diagnosis to do? Firmware upgrades, line tests?

[#2]

Try relevant links below.

[#3]

Thanks for these, nothing obvious in the information and checks I could do, I've reported as a fault for now. Because it's intermittent and it cannot be reproduced, I'm not feeling too hopeful.

[#4 · Accepted solution]

So in the end the speed issue was quiet simple although it took me lots of testing to get there. My devices were switching to the 2.4Ghz band, so I just split the two bands on the router settings and forced devices on to the preferred 5GHz or 2.4 Ghz bands .

The line fault check I raised previously came back negative.  I spoke with Plusnet on the phone, it was very much on the "we can't guarantee speeds on Wi-Fi" and they didn't really listen to my account of what was happening.  As as such they overlooked something simple an average homeuser can resolve.

As for the Wi-Fi/router resetting I'm yet to find what this was and I guess it could happen again...