This sounds a lot like the issues I had with my PlusNet FF service.
See the thread here:
https://forums.thinkbroadband.com/plusnet/t/4788018-...
The end result was the dropouts were caused by a faulty fibre splice from the original installation and a duff ONT. Openreach replaced both and it has been rock solid ever since.
In my case, PlusNet blamed the router, my cabling and anything else they could to avoid admitting there was a problem.
Notably, whenever they tested, they could see nothing wrong, because when they tested, the connection was up and running.
At least with Zen you don't have the problem of them blaming the router, since they provided it.
Cheers,
Axolotl