Im sure you figured it out by now but if not you need a NON visible SIM to do this 1. settings/mobile network/Sim/access point names/ delete what there and add
Name: Visible Internet
APN Type: default,dun,supl,mms
APN Protocol: IPV4V6
Mobile Virtual Network Operator Type: GID
Mobile Virtual Network Operator Value: BAE1000000000000
I didn't have a spare SIM card, so I had to go down a different route.
On the 21st, OnePlus released version C.44 which seems to prompt my phone to update to C.40. Once I realized what was happening - that the update said version 11 C.40 and I thought this was Android 11, I thought I would have to downgrade to Android 11 from Android 12.
But before I did that, I downloaded the Oxygen Updater App from the Play Store and had it check for any applicable updates for my phone and download them. It can't install it for you if you are not rooted, it will tell you if you are/aren't and provide instructions. The instructions won't work on C.40 though as it isn't technically OxygenOS anymore. So you will need to download the local storage updater app from OnePlus here: OnePlus Local Update.
Once I did this, all of the APN's listed above auto populated and mobile services had returned. Hope this helps others still running into this issue.