I've just factory-reset the printer, reconnected to the WiFi and it made no difference, so it is not a printer :-(

@Joseph-TP, thanks for looking into this. I have just tried auto-update, and there was no new version found. Here it the topology: [ISP Modem (192.168.1.1)] ----- (LAN Cable) -----...

I've upgraded my old TP-Link Archer router to new Archer BE930. I've set up the wifi to be exactly the same and mapped DHCP static leases to be the same as in the old router, so that I don't have to...