It could be that the tunneling service on n8n’s servers is having trouble reaching the tunnel target host (i.e. your running instance).
If you can run n8n in docker, you could follow those answers you found just to see if it works that way first.
If the n8n tunnel feature doesn’t work, even in docker, there are many other options for establishing a tunnel that allows those public-internet services to reach your n8n instance, so you may want to try one of them and follow their steps to set it up.
Here is a list of tunneling options which includes links and a brief description of each one.