MFC-J6510DW going offline both wired & Wifi after a few days with no print

[windbag]

I've had this printer for several years. Basically works fine, but has a recurring network disconnect problem.
When connected to the ISP (Virgin Hub3) by WiFi, it often disconnects completely after a few days inactivity.
It goes "offline", and requires a reboot of some sort to get it to talk to teh network again.
I took the effort to give it a wired ethernet input, and have been disappointed that it disconnects even more frequently in Wired mode.
Yes, I've raised it with Virgin, who, surprise, surprise, say
"as this is the only device that is having issues with service connection, it would suggest it is a third-party issue. Are you able to contact the printer manufacturer to see if anything can be done to resolve the matter? "
The Brother website I am able to access gives no "contact" method, and the FAQ's don't address the "offline" thing occurring at unpredictable times after inactivity.
I've checked the drivers, they are current, and I've tried the "network connection repair" utility. Very disappointing.
As it is currently back in WiFi mode, it asked permission to fix the WiFi IP address, but didn't warn me that it basically then rebooted the printer and deleted all the print queue - and took almost as long as a manual reboot to do it!
At least with a manual reboot it doesn't it doesn't lose the print queue, so for me, the utility is a backward step
It's not any firewall action causing this as far as I can see.

Any suggestions for anything I can tweak in the settings that still enable it to sleep (which is most of the time in my domestic setting) ?

 

[(vinny)]

does it have a fixed ip address

 

[windbag]

does it have a fixed ip address

No.
- partly because I've had to fix 4 others (not for disconnection reasons), partly because the Virgin Hub3 methodology to do it is flakey and awkward, partly because it's like having a dog & barking yourself, and partly because we've been in the 21st century for more than 2 decades. Though I suppose I could grit my teeth and try to set up TWO more fixed IPs, one for WiFi, one for Wired.
My experience in such efforts infers that Sod's law will win out and that, in a couple of weeks, I'll find the printer no different.

 

[windbag]

Anything else to try to stop this going offline ?