Have you tried mashing the escape button during the Lattepande boot? Based on these docs it seems like that should take you to the BIOS menu, where presumably you could then choose the boot device.
There are steps listed there for booting Ubuntu on the Lattepanda, so I would suggest trying to follow those steps exactly, except using PureOS instead of Ubuntu.
That said, I see in the Lattepanda documentation that it uses UEFI to boot, and I don’t believe PureOS supports UEFI since it’s not compatible with PureOS’s FLOSS-only philosophy, so that may explain your inability to boot PureOS from USB. It’s probably worth trying with Ubuntu to first verify that you’re able to boot Ubuntu successfully, and then if you can do that, but PureOS doesn’t work, it’s almost certainly a UEFI problem