The nRF91M1 PPP Host Application (91m1_ppp on nRF54L15 or nRF54LM20B) needs a separate Serial Modem image on the wired nRF9151 or nRF9151 SMA DK, configured for PPP + CMUX on UART0 for an external host MCU. Every 91m1_ppp bundle carries the upstream archive CI tested that night, so there is no separate asset to download, and on-target tests flash that same archive, so what ships beside the host build is what CI verified against it.
The exact tag varies by SMHA release. Each bundle's README.md names the Serial Modem version and links to the upstream release page. Browse Serial Modem releases for the source, or use the zip already in your download.
Besides the full-flash .hex, the archive holds the ELF with debug symbols, a Kconfig snapshot, the signed application and MCUboot slot images, DFU packages for programming without a debugger, and a devicetree snapshot.
Extract it, flash the .hex on the Serial Modem DK first, then flash the host bundle's merged.hex on the host DK:
unzip serial_modem_v<tag>_nrf9151dk_nrf91m1.zip
nrfutil device program \
--firmware serial_modem_v<tag>_nrf9151dk_nrf91m1.hex --recover
Replace <tag> with the version named in the bundle README.md (for example 2.0.0-preview3).
See nRF91M1 PPP Host Application's hardware setup for wiring and Board Configurator settings.