UPDATE - PROBLEM SOLVED:
It turns out it was a router issue. Yes, this is a home-server so port forwarding has to be set up in the router.
If you have a Netgear router and the port forwarding function seems to not be working as it should, check online in the Netgear community for a corresponding issue for your model. It took me several days of troubleshooting everything else assuming the port forwarding in my router was working fine. I had to downgrade (yes downgrade) several firmware updates and reconfigure all of the settings including port forwarding. The traccar server is happy now receiving all device communication.
Note that this is a relatively common issue for some Netgear routers which is absolutely ridiculous that Netgear can not get such a basic functionality correct.
Also for Anton,
on the Linux uninstall instructions (https://www.traccar.org/linux/), the command
sudo rm /etc/systemd/system/traccar
should be
sudo rm /etc/systemd/system/traccar.service
at least this is what works for Linux Ubuntu Server 20.04.
UPDATE - PROBLEM SOLVED:
It turns out it was a router issue. Yes, this is a home-server so port forwarding has to be set up in the router.
If you have a Netgear router and the port forwarding function seems to not be working as it should, check online in the Netgear community for a corresponding issue for your model. It took me several days of troubleshooting everything else assuming the port forwarding in my router was working fine. I had to downgrade (yes downgrade) several firmware updates and reconfigure all of the settings including port forwarding. The traccar server is happy now receiving all device communication.
Note that this is a relatively common issue for some Netgear routers which is absolutely ridiculous that Netgear can not get such a basic functionality correct.