The more I think about this the more I wonder about a problem outside your Draytek.

Cable from Draytek to ONT?

When you get to the router I assume all lights on the ONT are normal. In particular the power is on, LOS off, PON on and occasional flashes on the LAN light.

Is the Draytek accessable and does the Internet access say up or down?

I would also factory reset and configure from scratch.

This should be absolutely reliable.

My 2927 and the 2860 before it could go forever without a reboot.

Router is not accessible remotely obviously, but when I get there it is accessible and has been up all the while. The ONT looks fine and I don't have any suspicions about the PSUs. The draytek was absolutely reliable when it was on ADSL and ran for months on end without reboot or any problems. I had a look at the syslog which I have stored on my NAS drive just prior to the latest line-drop and noticed a few things that might have given a clue - can't remember what they were exactly and can't access it just now so will have to have a look when I regain access and I will try to post an extract. I don't remember what the Draytek said about the internet state last time I reconnected it but next time I will pay attention to that. It is my suspicion that the Plusnet system is sending something that they expect their router to respond to whereas my Draytek is not responding so they turn my link off hence my thoughts of using their router in bridge mode - are you sure that wouldn't help? I don't know much about bridge mode so am in the dark. Also its taken me a while to get the draytek set up as it is now and I am not sure I can remember how to reapply the settings after a factory reset so am reluctant to try that without trying other things first. Thanks for your help though (should have started with that, I am most grateful). I can't get back to the router until Thursday earliest so will try things then.

@SteveM84

If it's "accessible and has been up all the while" is the problem a remote access (VPN) issue?

If you have the Plusnet static IP address option (the one off £5) you can run a "ThinkBroadband" monitor to your router (enable PING from WAN from their address range).

The only thing that Plusnet "send" is TR-069 and the Draytek shouldn't respond to that.

The Plusnet Hub 2 "bridge mode" is really only any use replacing an Openreach modem on FTTC so not appropriate with FTTP.

As far as settings are concerened, last time I bought a new router I screenshotted all the settings on the old one and did their equivalents on the new one.

Is the problem that the external IP address changes?

I do have a VPN set up but the access that I cannot gain is via https to the router's login page in the first instance. Neither can I get VPN access of course. I suppose it could be the change of IP address but I would have thought the Draytek would handle that? Does it get informed of an IP address change? ( such a long time since I did my TCP/IP course in Nice ;) ). I had not realised fixed IP was only a one-off payment I will look into that. Reading your next post - screenshotting is a great idea I will do that when I next visit.

Another thing I tried was to set up a regular reboot in the hope that any disconnection would last maximum 24 hours but the scheduling doesn't seem to work as I would have hoped and has not rebooted the router at all (what I think I mean is I do not understand it).