Before all

Make sure the server is up to date and isn't using smod

Make sure the server has fully loaded, ie. the last console message should be "Waiting for players..."

Connecting

Hosted on the same PC the game is played on -> connect using localhost or 127.0.0.1 as the IP

Hosted on a different PC in the same LAN -> connect using the host PC's internal IP

Hosted on a VPS or something outside your LAN -> connect using the server's external IP

Config

ipv4_bind_ip: 0.0.0.0 - It's best to leave this as 0.0.0.0 but can be set to your machines LOCAL IP. If you have a VPS/VDS this can also be set to your external IP

ipv6_bind_ip: :: - ALWAYS Leave this setting as it is.

server_ip: auto - This can also just be left as auto however it can be set to your server EXTERNAL IP

"Still doesn't work"

Allow LocalAdmin (or MultiAdmin) through your firewall and/or antivirus

Port-forward it properly, see server-faq 18

Make sure there are no ghost processes running you can do this by looking for the in task manager (top/htop on Linux) or restarting the machine.

If you're currently running on an LTE network (mobile network connection) your ISP may be blocking certain connections. You can check with their support to see if this is the case.