I have an RS485 modem that I connected to my computer using a USB to RS485 converter. I can connect to it using Minicom with a baud rate of 9600, 8N1, and xonxoff flow control settings. The modem responds to AT commands.

I have tried configuring pppd and chat settings, but only the chat settings execute correctly.
I tried setting the MTU and MRU values between 550 and 1500, but unfortunately, I couldn’t get it to work with these values either.

I’m running out of ideas.
How can I get more meaningful logs to understand why I’m not getting a response?

sudo pppd call quectel-ppp
pppd options in effect:
debug           # (from /etc/ppp/peers/quectel-ppp)
kdebug 1                # (from /etc/ppp/peers/quectel-ppp)
nodetach                # (from /etc/ppp/peers/quectel-ppp)
dump            # (from /etc/ppp/peers/quectel-ppp)
noauth          # (from /etc/ppp/peers/quectel-ppp)
/dev/ttyUSB0            # (from /etc/ppp/peers/quectel-ppp)
9600            # (from /etc/ppp/peers/quectel-ppp)
lock            # (from /etc/ppp/peers/quectel-ppp)
connect /usr/sbin/chat -s -v -f /etc/ppp/peers/quectel-chat-connect             # (from /etc/ppp/peers/quectel-ppp)
disconnect /usr/sbin/chat -s -v -f /etc/ppp/peers/quectel-chat-disconnect               # (from /etc/ppp/peers/quectel-ppp)
xonxoff         # (from /etc/ppp/peers/quectel-ppp)
local           # (from /etc/ppp/peers/quectel-ppp)
mru 1492                # (from /etc/ppp/peers/quectel-ppp)
mtu 1492                # (from /etc/ppp/peers/quectel-ppp)
lcp-restart 6           # (from /etc/ppp/peers/quectel-ppp)
novj            # (from /etc/ppp/peers/quectel-ppp)
novjccomp               # (from /etc/ppp/peers/quectel-ppp)
ipcp-accept-local               # (from /etc/ppp/peers/quectel-ppp)
ipcp-accept-remote              # (from /etc/ppp/peers/quectel-ppp)
noipdefault             # (from /etc/ppp/peers/quectel-ppp)
defaultroute            # (from /etc/ppp/peers/quectel-ppp)
usepeerdns              # (from /etc/ppp/peers/quectel-ppp)
noccp           # (from /etc/ppp/peers/quectel-ppp)
abort on (BUSY)
abort on (NO CARRIER)
abort on (NO DIALTONE)
abort on (ERROR)
abort on (NO ANSWER)
timeout set to 120 seconds
send (AT^M)
expect (OK)

^M
OKOK
 -- got it

send (ATE0^M)
expect (OK)

^M^M
OKOK
 -- got it

send (ATI^M)
expect (OK)

^M^M
Quectel
Quectel^M
EG915N
EG915N^M
Revision: EG915NEUAGR03A09M16
Revision: EG915NEUAGR03A09M16^M

^M
OKOK
 -- got it

send (AT+CSQ^M)
expect (OK)

^M^M
+CSQ: 22,99
+CSQ: 22,99^M

^M
OKOK
 -- got it

send (AT+CPIN?^M)
expect (OK)

^M^M
+CPIN: READY
+CPIN: READY^M

^M
OKOK
 -- got it

send (AT+COPS?^M)
expect (OK)

^M^M
+COPS: 0,2,"21630",7
+COPS: 0,2,"21630",7^M

^M
OKOK
 -- got it

send (AT+CGREG?^M)
expect (OK)

^M^M
+CGREG: 2,1,"2EE1","B6F06",7
+CGREG: 2,1,"2EE1","B6F06",7^M

^M
OKOK
 -- got it

send (ATZ^M)
expect (OK)

^M^M
OKOK
 -- got it

send (AT+CGDCONT=1,"IP","internet.telekom",,0,0^M)
expect (OK)
AT+CGDCONT=1,"IP","internet.telekom",,0,0
^MAT+CGDCONT=1,"IP","internet.telekom",,0,0^M^M
OKOK
 -- got it

send (ATD*99#^M)
expect (CONNECT)
ATD*99#
^MATD*99#^M^M
CONNECTCONNECT
 -- got it

PPPD

Using interface ppp0
Connect: ppp0 <--> /dev/ttyUSB0
sent [LCP ConfReq id=0x1 <mru 1492> <asyncmap 0xa0000> <magic 0x16ea0a07> <pcomp> <accomp>]
sent [LCP ConfReq id=0x1 <mru 1492> <asyncmap 0xa0000> <magic 0x16ea0a07> <pcomp> <accomp>]
sent [LCP ConfReq id=0x1 <mru 1492> <asyncmap 0xa0000> <magic 0x16ea0a07> <pcomp> <accomp>]
sent [LCP ConfReq id=0x1 <mru 1492> <asyncmap 0xa0000> <magic 0x16ea0a07> <pcomp> <accomp>]
sent [LCP ConfReq id=0x1 <mru 1492> <asyncmap 0xa0000> <magic 0x16ea0a07> <pcomp> <accomp>]
sent [LCP ConfReq id=0x1 <mru 1492> <asyncmap 0xa0000> <magic 0x16ea0a07> <pcomp> <accomp>]
sent [LCP ConfReq id=0x1 <mru 1492> <asyncmap 0xa0000> <magic 0x16ea0a07> <pcomp> <accomp>]
sent [LCP ConfReq id=0x1 <mru 1492> <asyncmap 0xa0000> <magic 0x16ea0a07> <pcomp> <accomp>]
sent [LCP ConfReq id=0x1 <mru 1492> <asyncmap 0xa0000> <magic 0x16ea0a07> <pcomp> <accomp>]
sent [LCP ConfReq id=0x1 <mru 1492> <asyncmap 0xa0000> <magic 0x16ea0a07> <pcomp> <accomp>]
LCP: timeout sending Config-Requests
Connection terminated.
abort on (ERROR)
abort on (NO DIALTONE)
send (+++ATH^M)

I looked at the following forum posts:
https://forums.quectel.com/t/quectel-ec25-ppp-connect-failure/3379/6
http://web.mit.edu/~linux/redhat/docs/untarred/PPP-Client-HOWTO-13.html

quectel-ppp

noauth
connect '/usr/sbin/chat -s -v -f /etc/ppp/peers/quectel-chat-connect'
disconnect '/usr/sbin/chat -s -v -f /etc/ppp/peers/quectel-chat-disconnect'
debug
/dev/ttyUSB0
9600
defaultroute
noipdefault
novj
novjccomp
noccp
ipcp-accept-local
ipcp-accept-remote
local
lock
dump
kdebug 1
nodetach
# Hardware flow control
#nocrtscts
# Software flow control
xonxoff
usepeerdns
lcp-restart 6
mtu 1492
mru 1492

/etc/ppp/peers/quectel-chat-connect:

ABORT 'BUSY'
ABORT 'NO CARRIER'
ABORT 'NO DIALTONE'
ABORT 'ERROR'
ABORT 'NO ANSWER'
TIMEOUT 120
ECHO ON
'' AT
OK ATE0
OK ATI
OK AT+CSQ
OK AT+CPIN?
OK AT+COPS?
OK AT+CGREG?
OK ATZ
OK AT+CGDCONT=1,"IP","internet.telekom",,0,0
OK ATD*99#
CONNECT