DNS on camera doesnt affect anything if the camera is connected to PoE port of an NVR. At least in shouldn't. As actual access via internet to that camera happens via NVR.
You could upgrade to 5.7.19 and see how it goes
Well i wouldn't, its a lot of changes. Here is a list of firmwares in between for your model
I would go firstly on 4.1.10, then the latest
https://www.hikvisioneurope.com/eu/portal/?dir=portal/Technical%20Materials/02%20%20NVR/00%20%20Product%20Firmware/04%20I-series/%5B76%2077%2096%20NI-I%5D