Hi Anders,
In that case, the Macrium Rescue media will be needed. You should be able to access the NAS from the rescue media, and this documentation should help.
Accessing network shares in Windows PE - KnowledgeBase v7.2 https://knowledgebase.macrium.com/display/KNOW72/Accessing+network+shares+in+Windows+PE
I'm not sure if the settings are the same in ReflectX (the latest version, which is subscription-based). I'm using the grandfathered edition, I think v8.x (free).
Resolving Rescue Media Network Issues - Macrium Reflect Knowledgebase: https://knowledgebase.macrium.com/display/KNOW72/Resolving+network+issues+in+Windows+PE You can also copy the backup images to the USB disk and access them if the NAS is unreachable in a worst-case scenario.
The backup images look like this. The larger ones are full backups, and the smaller ones are incremental backups. I'm retaining the full backups for 12 weeks, just in case. That's why a lot of backup images in that folder.
You can put them in a folder on your USB disk and access them from the Macrium Rescue WinPE environment.