The Mate 20X is a 3G voice phone. Even though it is a 4G LTE capable phone it is not a VoLTE/HD Voice phone. Hence the reason why this phone was identified as a 3G device. So basically when the cutoff date comes in February, the phone will not be able to make phone calls or receive texts. Technically it should still be capable of using the 4G data but take this with a grain of salt because they basically blacklisted the IMEI/Model of this phone. So they could just force these to not work anymore.
FYI (and I know this is really a late reply), this is incorrect if you were lucky enough to have unlocked the bootloader on your Mate 20 X (EVR-L29) phone (or if you can do so if the opportunity ever arises again in the future).
Thanks to someone else on this forum (posting about a different huawei phone, I've lost the original poster), I've discovered that it IS in fact possible to enable VoLTE on AT&T (ATT) if your phone is rooted.
Before making these changes attempts to make or receive calls will fail even though data and texts will be working and there is a 4g signal symbol.... and going into settings -> wireless & networks -> mobile network will not have the necessary "VoLTE calls" option to turn VoLTE on.
Simple way to enable VoLTE on rooted Mate 20 X on AT&T:
adb shell
su
mount -o rw,remount /
echo "ro.config.hw_volte_on=true" >> /system/build.prop
echo "ro.config.hw_volte_dyn=false" >> /system/build.prop
mount -o ro,remount /
Restart your phone.
The "VoLTE calls" option will now show up, and can be turned on... after which phone calls will again work on the Mate 20 X on AT&T.
I hope this helps someone like it helped me to figure out, even though I know unlocked bootloaders are rare on this phone.
I also couldn't find another phone this size that would work on AT&T (aside from fold ones that I don't want) so really happy that this ended up working.