The Serial Modem bundle is *_nrf9151dk_nrf91m1.zip. The host UART is the nRF91M1 pinout on UART0 (P0.27 TX, P0.26 RX, P0.14 RTS, P0.15 CTS). DTR is on P0.31 and RI on P0.30.
Until the nRF9151 DK Board Configurator can route DTR automatically, the modem DK also expects P0.31 (DTR) jumpered to GND when you use a PC host. An external MCU host should drive DTR from its GPIO instead (as this application does through P1.11).
CI tracks the newest upstream release that ships the nRF91M1 bundle. Static settings such as console_baudrate live in tests/on_target/ci/serial_modem_firmware.yml. Set SERIAL_MODEM_RELEASE to pin a specific tag locally.