MR6110 unable to connect to Telstra

Bought the MR6110 from JB Hi-Fi in Tweed City (BTW great service there) but after just 3 days I've experienced "Unable to join the network [SSID]" on multiple devices (iPhone, iPad, sometime after being connected for 30+ minutes. 
Disconnecting and forgetting the SSID then reconnecting seems to work, but it's a pain and I didn't have this problem with the ZTE MU5120 from the same location using the same Telstra 5G network. 

I only changed routers because JB Hi-Fi offered a better deal than I had with Telstra on the ZTE device.  I contacted Netgear Support but the chat session offered only that I should contact Telstra, which anyone in Australia knows will take up most of the day and generally achieve nothing. 

I have been able to connect to the admin page and checked that the firmware is NTGX65_12.01.41.00.  Netgear chat suggested changing the Channel & DNS.  From the www. routerlogin. com/ interface Status/diagnostics menu I see that the channel is 3148 but there's really no way to change that, the only thing that's configurable is 5G, 2.4Ghz or both.  I use 5G only.

If this continues I still have the ZTE MU5120 and I'm wondering if I can swap the sim into that?