There are a lot of heavyweights answering here but I would still like to chime in since I faced a similar issue with 1 public node and 2 private nodes.
For the public node, confirming UDP port was open in router and restarting update node list service in asl-menu diagnostics section fixed it for me.
For the private nodes, I added the node number of the private node on asl3 in the node I wished to connect to which fixed it.
You could also try it with your setup, maybe it might solve this. In your 42610 asl-menu, add a hub/radioless node 42393 without a password. If it fixes the issue, then the dev team could possibly see whats up with all of this.
Mine would also connect for a second and then hangup. Try both, first the public node way, if that does nothing then the second way.
Yesterday, I suggested that this was looking like a networking issue and asked about firewalls in the server/node, firewalls in the router, whether you are using any tunnels, VPNs, or the like.
The ASL3 Pi Appliance, by default, includes a firewall with a set of rules. Those rules allow UDP traffic for ports 4560-4580 (IAX's default port 4569 and a few before/after). Your port 4888 is outside that range. Did you update the firewall configuration on your system?
I also installed checked the UDP WAN IP with a stun client to use google's stun server. 'Bottom line there is: both UDP and TCP IP's are the same; so, no issues like T-Mobile .
Also, no tunnels / VPN being used here. I also am using a nonstandard SSH port and it is open for TCP through firewalld. Of course, 4888 is open for UDP.