Considerable care is required when exposing systems to the Internet and you will want to be careful what you allow.
The basics are that you will need to create a path from the Internet through to your Pi. Normally this is blocked by your router for safety. So the starting point is to create a NAT/PAT entry in your router that allows a connection to your external (ISP provided) IP address on port 1880 to connect to the internal IP address of your Pi also on port 1880.
Once that works, you can, if you like, make the external address anything you like (port 80 for example so that people don't have to mess with port numbers).
If you have a NAS, you might be able to use the reverse proxy feature on that to provide a connection too with the advantage that it may already have https configured without you needing to set that up on the Pi.
There are some additional notes on safety and security
here if you need them.