You CAN NOT just tick the AnnexM box and wait for the upload rate to magically increase itself.
I had asked StrikerX a year back or so if I could do this, and that was before it was being beta tested even so he said it depends on your exchange.
Now, if you want AnnexM, you're going to have to request it by messaging PTCLCares and then wait for the modem to re-sync at AnnexM with the higher upload rate, while slightly compromising on download rate.
TL;DR Ticking the box will do nothing if your exchange hasn't enabled it. It'll just cause your modem to disconnect.
Sent from my SM-G935F