Based on the Magisk module found here: https://forum.xda-developers.com/t/signal-apn-fix-magisk-mod.3799266/#post-76708352
Updated with APN config from here: https://github.com/BeYkeRYkt/android_device_google_wahoo/blob/aosp-new/master/apns-full-conf.xml
This was the most complete and update APN config I could find. There's a better one for chinese carriers that uses the latest Xiaomi config that may work better for some, check it out here: https://github.com/RiwiHow/FixChinaCarrier