Firmware on both cams are updated to the latest, but the issue occured before and after. Distance between the router is very small, only a few meters, not more than 4 or 5 I recon. Only thing between the cam and router is an old brick wall and wooden floorboards. The signal is strong, in have used several apps to test it on my iPad as well.
The new router has a dual band wifi again, seems quite standard these days. I do notice that older devices, like the cams, only connect to the 2,4GHz and when I would accidentally connect my iPad to it, the comunications are slow. This might be a reason for it and also why the old router had no issues working on only 1 band.