HDD for long term storage. More reliable, has a higher number (essentially infinite assuming the drive never fails) of read/writes before failing. Cheaper and higher capacity than any ssd or m.2. Also if you dont keep applying a small electrical charge to an m.2 they eventually lose the data. HDD doesnt really lose data as easily. Also data recovery is easier with HDD. Finally you know when a HDD is on its way out as it will show slower write speeds and become noisier etc.
I used to work in a service desk looking after maybe… 4000 desktops and 2000 laptops for a hospital and the amount of ssd and m.2 failures we had was very costly.
Do your access rules work in both directions? Do you have any strange routing going on? Do you need to configure a static route for the returning traffic?
Sorry, knowing very little about your setup means i can only suggest vague possibilities based on networks i work with.