Same happened to me. My LG 8X was working fine with red pocket then I updated. I believe it moved to android 11 and now it's really whacky. as soon as i put in the red pocket att apns the phone just says "call ended" (my screen also blacks out till i hit the power button). Also, this afternoon, my work landline could not even call my LG 8X, would just hang up. RPM support was worthless. The only way i can make or take phone calls is to remove the apn.
I'm thinking of doing a factory reset and if that doesn't work, I'm researching into flashing back to Android 10. But that's scary.