Can't boot from M.2 SSD

My fiancée has a Dell laptop. We got an M.2 SSD drive to use as a primary drive for her booting operating system, and 2.5" 1tb for storage. We couldn't get the M.2 to boot when we put the formatted 2.5" drive inside the laptop, so thought we'd try adding an operating system to it, but when we went to boot sequence, the M.2 SSD won't show and just boots into the new operating system instead that we have just put on. We are very frustrated so would appreciate some professional feedback to get it sorted, as it has done my coconut in all afternoon trying to sort it out she is using Windows 10.

I thought we could put a formatted 2.5" drive in the laptop to use for storage and use the M.2 as the boot drive, but the laptop has other ideas.

Thanks in advance for any advice.