I am also using 4G LTE on Vodafone through a TP Link router, speed has been good not had any problems for the last 5 months and then this issue appeared. I have port forwarding set but cannot access the web portal of the camera or router, I guess the service provider is blocking ports but hik-connect allows a connection but cannot make remote configuration changes. My whole system is restarted most Saturdays as the area where the camera is located have frequent power outages for maintenance so no intervention is currently required ;-)
Noip.com doesn't seem to work for me although i do get a IP, I was in a rush to setup the camera on my last day in india so setup NoIP on the router and setup hik-connect on the camera, I may have done it wrong was hoping to configure the camera remotely from the UK but haven't manage to that. I did port forward the router for remote management and port forwarded the camera service ports.
Strange thing is the camera emails via gmail, so I can get the router IP, NoIP give an IP and Hik-connect also gives an IP but all three are not always the same. I have tried all three IP's without success.
Only way I can think it is possible is to get someone with a laptop to connect to the local network and teamviewer in to make changes, only issue is security.
I think in my case the camera doesn't seem to be down as I am still getting emails when I cannot Hik-Connect, it could be as you suggested a internet speed issue. If I manage to connect to the web portal I will post back here.