I haven't used the hik app, but are you sure you've got the correct password entered in the app's 'config' for the new cam?
just fishin, at this point, but if your phone is coming in from the internet (not on local wi-fi) then you must have poked a hole in your router/firewall
to reach your cam. Perhaps you need to repeat that process for the new cam in order to reach it from outside the local network?
another possibility is that the previous owner had changed the access ports around on the cam such that the app can't
connect to it on what used to be the default port. If you can post a shot of the cam's network config screen from the
web browser interface, someone might be able to check/confirm this...