@SidneyReilly . Look at this thread. It has URLs which might give access to the wifi settings on the 5G modem. The idea is to change the SSID names to be different from each other. It might allow the printer to connect. The extender might have had the same problem as the printer.
If the URLs don't work, do factory reset on modem. Login immediately and make wifi changes, including security settings. Then PPPoE settings.
BTW, is there any setting relating to VLAN ID, like on regular routers?
Have you had occurrences of the throughput decreasing after some duration of usage?
Does the printer have a USB or ethernet connection, or just wifi?
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