PPP with SIM800: `lcp_rx: We only support default or all zeros for ACCM`

The device (ESP32, TTGO T-Call) is not getting an IP address when using PPP.

Enabling `CONFIG_DEBUG_NET_INFO` gives me: `lcp_rx: We only support default or all zeros for ACCM` which repeats indefinitely.

This [article](https://acassis.wordpress.com/2019/02/10/debugging-pppd-using-quectel-m95-on-samd21-board/) suggests adding `AT+QACCM=0,0` to the modem init chat, but this is a Quectel-specific command.
On SIM800, it should correspond to `AT+IFC=0,0`, but adding `"OK AT+IFC=0,0 "` to connect_script after `ATE1` didn't help (still have same error).

Adding `bptr += 4;` patch to `lcp.c` as suggested [here](https://acassis.wordpress.com/2019/02/24/accessing-internet-from-nuttx-ppp-connection/) helps, but I'm not sure if it has any side-effects.