4G SIMs usually don't have a fixed IP address and often use shared IP addresses so you can't do port forwarding anyway. For Gmail you have to set a device password rather than using your normal Gmail login details. I tested this just a few weeks ago and it still works.
Hi,
I've set up email notifications on Hik NVRs with no issues, although this was using Gmail. What are you using for your email? I might be able to test and confirm it work.
Cheers,
Mark