repeater not connecting to gateway

It makes sense to me. Depending upon the Canon printer model and the age of the printer the network adapter capability is probably part of the equation. 

Assuming you have the Arcadyan or Sagemcon gateway you need to use the T-Mobile home internet mobile application to login as “admin” and be able to make changes.

Once you are logged in you will want to select Red on the bottom bar

You should see a magenta dot with a white + in the middle. (select that)

You can “ADD” a network SSID for your printer on the 2.4 GHz frequency (pass phrase as well)

Provide it with WPA/WPA2 (AES)  for the encryption authentication type

Once you have the network name/passphrase/authentication type then “Apply/Save

After that try to connect your printer to the network name you set up for the printer. My guess is the printer only supports 2.4 GHz radio frequency AND will only support WPA or WPA2 authentication type. 

If the default for the 2.4 GHz network for every thing else is WPA2/WPA3 (AES) then the printer cannot connect as it is expecting WPA for authentication type IF it is 5-7 years old. High probability. If you had your printer connected with an Ethernet cable then remove that before you attempt to connect to the new network name with the WPA authentication type.