Canon MB2100 Series Online Manual page 869

Make sure printer is not too far away.
Check6
Locate your smartphone/tablet close enough to communicate with the printer.
Check for obstructions.
Check7
Wireless communication between different rooms or floors is generally poor. Move the printer as necessary.
Make sure there is no source of radio wave interference nearby.
Check8
If a device like a microwave oven that emits radio waves of the same frequency bandwidth as a wireless router
is nearby, it may cause interference. Place the printer as far away from the interference source as possible.
Make sure 5 devices are already connected.
Check9
Direct connection does not allow more than 5 devices to be connected.
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