Switch from RNDIS to ECM with AT+QCFG="usbnet",1
Configure the APN with at+qicsgp=1,1,"your apn"
Check the context activation status with AT+QIACT?
“Dial” with at+qnetdevctl=1,1,1
Then run DHCP client on the interface associated with the modem, it should be mentioned in dmesg next to cdc_ether.