I see there are many posts regarding the AC800S modem, which I have purchased recently. I also see Optus is not responding to most questions about this on the Yes Crowd forum. Clearly if it is such a known issue of unreliability and restarting then Opts should stop selling it and provide some recourse to those of us that have purchased one. More importantly who are reliant on this for internet. Optus what do you plan to do about this? - it is severely hurting your brand, particularly as it seems no one cares much at Optus? I will post this to the Optus FB and Twitter pages as well
Lots of posts of unreliability on this forum don't mean anything certain, since anyone with a problem will come here. However, their last modem, the Netgear 790S/Telstra was a pile of junk containing obvious design flaws that they would never answer to. Optus staff not answering on the topic of wireless modems is nothing new, since they all have landline and know nothing about them.
The reason they do this to us is that there aren't all that many hotspot alternatives that do cat 9-11. Most people are using their Samsung 7-9 or iPhone 7-10. The ZTE modems are reliable, but only Cat 6 and on the edge of bankruptcy, due to being fined 1.2bil. Huawei's serious modems aren't mobile.
Your best course of action is to get a replacement.