The BM818-E1 does support 1800 MHz (at least the most common variant, which is 3gpp band 3).
If the confidential datasheet that Purism has doesn’t mention this, it is likely a mistake in that datasheet and not in the website (which explicitly mentions it), since B3 support is basically a hard requirement for a EU/International variant that does dual-band GSM (which is 900 MHz B8 and 1800 MHz B3).
In Singapore, US surveillance isn’t the major issue, Singapore itself is one of the more surveillance-intensive states and the government itself is known to keep taps on communication in the country.
Of course there are still US interests, and also Chinese interests.
The BM818 combines both, since the Chipset and most of the software comes from Qualcomm in the US, and the other parts of the software (operating system services and some more) from Broadmobi.
The Qualcomm vulnerabilities are nothing unexpected, every modem manufacturer will have them, since they all run very complex software.
These vulnerabilities will themselves not directly apply to the Librem, because it doesn’t use the Qualcomm-integrated stack and so cannot be attacked by the same chained exploits.
However, the “Respects Your Freedom”-badge from the FSF, which Purism strives to achieve, prevents Purism to ship firmware updates for the modem, since that would require Purism to ship non-free firmware.
This could lead to future exploits at least for the modem portion (though the OS is unlikely to be attackable from that angle).