Thanks TPG, all fixed now. No definite cause determined but in the process of testing it somehow got fixed (maybe a loose terminal in the box at the basement).
For others interested, from today's experience, it seems that if the issue is not external to your building (i.e. the link from TPG to the box in the building) then you maybe left with a problem... . Not much I can add of value except for when considering to use this type of service - beware if it is internal wiring of your building you will be left with a problem you need to fix and maybe pay for and it is close to an impossible and expensive task to fix...
We've managed to check the update on your escalated case and it looks like the onsite technician found the problem at the Main distribution frame and fixed it by re-terminating the connection.
Thanks for raising this matter and giving us the opportunity to help you. Please don't hesitate to reach out should you need any assistance with your TPG service.