How to have intermittent signal issues investigated?

Hi, looking for advice from the community or any mods who can assist more directly.

I've been on M500 without much issue for approx 12 months until earlier this month since when I've regularly experienced some fairly prolonged periods of the internet being unusable - typically 30 mins but sometimes 12 hours+.

During these times the internet is not completely down but requests for web resources typically will timeout.  The issues coincide with "SYNC Timing Synchronization failure" error logs and very high packet loss according to BQM tool - for example here is my BQM for past 24 hours.

During these sync issue episodes, the online network status tool will sometimes acknowledge intermittent signal in your area, but sometimes no problems found.  The automated line checker phone service does not seem to ever acknoweldge a problem. 

On Friday, I was able to speak to an advisor to report the problem during a period of sync problems.  After taking me through a reset procedure which did not resolve any problems, she told me she would escalate my problem as she could not see any issues.  I was sent a leaflet by email relating to wifi setup.  I'll be honest, I did not leave that call feeling confident that the advisor had understood what was happening nor that they had passed on the issue correctly accurately.

My question is, how should I best proceed in getting this ongoing issue investigated and/or recieve updates on progress as the current quality of my line is not sustainable.

I appreciate any help or advice.  Thanks for reading!