This particular issue in the post relates to the NI-I series NVR, I believe ours actually shipped with the V4 firmware so even after switching to the oldest possible firmware, still hasn’t resolved it for these NVRs it seems!
Ive usually had to just factory default and they have all worked using the EUI version. Hiks firmware is a bit of a hit and a miss at the moment. Every firmware upgrade i would recommend a full backup and restore back to default.
I just had this exact same problem on a DS-7608NI-K2 /08. Firware was updated from from v3.04.104 to v4.30.060, after which the Hik-Connect service was offline. After trying a few differnt things I finally decided to just reconfirm the original settings on the NVR. I went to the Platform Access page and unticked the enable box, then ticked it again and saved the settings. Hik Connect then came straight back online.