Nighthawk M1 disconnects from network a dozen times a day

Hi All, 
I have a Nighthawk M1, firmware NTG9X50C_12.06.11.00 which I've been using as home internet (wfh) for the last 16 months. 

The first year has been amazing. The only dropout i experienced was in the morning I found I had to restart the router to put it back on the network, and then I'd be good to go. This modem has a dedicated sim card & data package. And up until recently I've been thrilled with the setup.

I also have a Iphone 13 max with a data allowance, and I datashare between the two devices as/if required.

In the last 4-5 months I've noticed the rate of dropouts has been increasing. For some reason, the drop outs and signal strength only really affects the M1 and not my phone. Whilst  I experience the odd dropped call, I find that i can hotspot off my phone all day without a problem. In fact, as I write this, I'm hotspotting off my mobile phone on my work computer. Whereas I would have thought that both the phone and the router were accessing the same tower and thus experiencing the same connectivity issues? 

I've gone into the settings, I've tried moving the router higher. I've change signal range etc, but aside from playing around with channels / networks and bands I'm really not sure what else to do. Can anyone offer advice? I'm in australia and accessing the Optus network, and as I said - The setup HAS worked perfectly, so I'm not sure what has changed. I've wondered if I should be adding antennas, but i wouldn't even know where to start.

If anyone has any advice regarding specific settings for the M1 on the optus network, I'd be happy to hear it.