Hub 3 5g frequent disconnections

Hi, since joining a little over a month ago I have been experiencing frequent disconnections. It’s been driving me crazy as I work from home. I have taken it upon myself to work through as many fault finding steps as I can but none of the following I have tried has worked.

1. disable “channel optimisation” box
2. rename the 2.4 & 5 GHz network ssid's. 
3. Only three devices connected to 5g now is my phone, PC and iPad. 
4. used airport utility to analyse congestions on channels. There were congestions, I have reassigned both 2g and 5g to an unused channel.
5. when i get a disconnection, I check signal strength (always good) and check my other device assigned to use 5g and that device also cannot connect.

Has anyone else got any further ideas on what to try? If nothing else I would like to know how I can book an engineer to resolve or better still switch for a hub5 and spare the engineer visit and more time sunk into this. I had a hub 5 previously in the same house and never had an issue!