Internet Intermittent/slow

This is Deja Vu - I make no apologies for using most of the post I made 2 years ago (with some amendmants)

Hello - is there anybody out there who can advise me.  I have been with Virgin for around 4 years. I am in CB3 Cambridge. Have a Hub 3 router, which was not great, so, around 3 years ago, following advice on this very forum, I purchased a TP Link Deco AC 1300 Mesh, with 3 nodes. I turned the router into a modem, fitted the Deco, with excellent results, speed up, dead spots no more - until 2 years ago. I had very slow, intermittent broadband. I went through the online help, rebooted and reset, which did not made any difference. I called the help desk . All they could tell me was that there are no reported issues, and that, as far as they were concerned, there are no problems with them. When pressed the (nice) young lady stated that as I was using third party equipment (Deco) Virgin would not engage with unless I disconnect the Deco, and turn the Hub 3 back into a router. I just knew that this would not make any difference.

Then, after 3 days of intermittent internet, out of the blue, the problem dissapeared, and all was good.

However, it has happened again. Exactly the same issue - and, not long after I signed a new 18 month contract.

Nothing has been changed/altered at this end. As it is intermittent, it is fine when it is working properly.

Any ideas/insights ?

Thank you

Clive