You can use supersu SuperSU v2.79 SR3
https://xdaforums.com/apps/supersu
Modem 21C (- 21C (13.2-00002) - download - mirror - official (latest, recommended on 5.x.x) here:
https://xdaforums.com/lg-g3/general/radio-modem-lg-g3-t3110376
The order of flashing should be this:
1º: modem 21C
2º: rom
3º: gapps
Restart system and configure
if you want root, after flash gapps (without restart):
4º: magisk or supersu for root
5º: mr_bump (needed if you use magisk or supersu)
After flashing mr_bump the phone automatically restarts once or twice (it's normal)
now configure