Nighthawk M6 5G Wifi Router connection Issues

[Gen3v15]

I have the above router, which, up until 3 days, operated and connected to perfectly to the network.
On Wednesday I had no connection at all but when it took it into the Telstra shop to get looked at, it worked perfectly! Good 5G connection. This happened again yesterday and again today. I had it on with a 5G connection all the way home but now I'm home I have no connection again.
This despite being able to get a 4G and 5G (2 bars) on my phone.
Any ideas as to what may be wrong?