Wi-Fi Printing Problem

We recently attended a trouble call where a customer was having intermittent problems printing to her Canon Wifi network printer.
She had been using it successfully to print from her Windows 10 laptop; her Apple iPhone and Apple iPad.
Suddenly one day, the printer stopped printing from any device.
Upon arrival at the customer, I rebooted the printer and rejoined the printer to her Wi-fi network, which was delivered by a Nokia ONT (Optical Network Terminal) provided by her fibre company.
Suddenly the printer sprang into life and started printing some spooled documents.
She then copied a private document on the printer itself and shortly thereafter, found that the Wi-fi printing at once again stopped.
I again checked the Wi-fi settings on the printer which showed no problem.
As part of my troubleshooting, I temporarily used my Smartphone as a Wi-fi “Hotspot” and joined all devices to it. The printing worked happily for 30 minutes via my Smartphone’s Hotspot. This showed me that the cause of the problem was her ONT.
I then rejoined the devices to the printer and the same problem ensued.
Seeing that the printer also had an Ethernet port, I plugged the printer directly into the ONT’s LAN port and the printing started again. After further testing, I concluded that there must be some incompatibility with the firewall and ports inside the ONT that was causing problems for the printer network data traffic. I could not conduct further testing of the ONT since her fibre company had removed administrator access.
It is always better to use a cable connection wherever possible.