One more idea:

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(From the manual of the compiler).

In some cases, the transmission rates of SEROUT2 instructions may present characters too quickly to the receiving device. It may not be desirable to use an extra pin for flow control. An optional Pace can be used to add character pacing to the serial output transmissions. This allows additional time between the characters as they are transmitted. The character pacing allows a delay of 1 to 65,535 milliseconds between each character transmitted.

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Maybe too quickly for the autobaud feature of your modem. Try to use the option PACE of the serout2 command.

Or just for a quick test instead of the PACE option:

serout2 TXD,396,["A"] 'Send A to modem
pause 100
serout2 TXD,396,["T"] 'Send T to modem
pause 100
serout2 TXD,396,[13] 'Send <CR> to modem
pause 100

Remember that autobaud is enabled only in command mode. When you go in data mode autobaud is disable.

Regards,

Luciano