WR21 and PPPD in Linux


We’re trying to connect to modem via either serial port – RS232 or RS485, but we couldn’t find any resources on Wiki on that topic.
Our connection diagram is: Linux board <-> Rs232(or 485) <-> WR21 <-> Service Provider <-> Internet.
We used serial port 0 and was able to see some Byte received and sent from that port whenever we execute a command like pppd on a Linux board.

We’ve tried so hard to search for the chatscript that works for WR21 with no luck. In the Event Log we see this:

CMD 0 Error Result: ATD99**1#

I’m curious which number we should configure in the chat script? We want to connect to ISP in Canada(TELUS) and US(Verizon or AT&T).

Can you please suggest any guideline that we need to follow up? We prefer to use RS485 port, but the RS232 should also be very helpful.

Looking forward to hearing from you soon.

Thanks in advance,