Problems with PPTP connection on cellular

[JDE100]

Trying to connect to my AX88U with latest Merlin firmware from a cellular connection. With no luck. Please see below log messages. Any ideas what the issue is? I can connect with same PPTP connection over wifi no problem.

Oct 7 17:27:37 pptpd[18159]: CTRL: Client 172.58.xx.xx control connection started
Oct 7 17:27:38 pptpd[18159]: CTRL: Starting call (launching pppd, opening GRE)
Oct 7 17:27:38 pptp[18160]: Plugin pptp.so loaded.
Oct 7 17:27:38 pptp[18160]: PPTP plugin version 0.8.5 compiled for pppd-2.4.7, linux-..
Oct 7 17:27:38 pptp[18160]: pppd 2.4.7 started by admin, uid 0
Oct 7 17:27:38 pptp[18160]: Couldn't allocate PPP unit 10 as it is already in use
Oct 7 17:27:38 kernel: pptp1: renamed from ppp11
Oct 7 17:27:38 pptp[18160]: Using interface pptp1
Oct 7 17:27:38 pptp[18160]: Connect: pptp1 <--> pptp (172.58.xx.xx)
Oct 7 17:28:08 pptp[18160]: LCP: timeout sending Config-Requests
Oct 7 17:28:08 pptp[18160]: Connection terminated.
Oct 7 17:28:08 pptp[18160]: Modem hangup
Oct 7 17:28:08 pptp[18160]: Exit.
Oct 7 17:28:08 pptpd[18159]: CTRL: Client pppd TERM sending
Oct 7 17:28:08 pptpd[18159]: CTRL: Client pppd finish wait
Oct 7 17:28:08 pptpd[18159]: CTRL: EOF or bad error reading ctrl packet length.
Oct 7 17:28:08 pptpd[18159]: CTRL: couldn't read packet header (exit)
Oct 7 17:28:08 pptpd[18159]: CTRL: Fatal error reading control message in disconnect sequence
Oct 7 17:28:08 pptpd[18159]: CTRL: Client 172.58.xx.xx control connection finished

 

[L&LD]

What 'cellular connection' (client device) are you using to connect with?

 

[JDE100]

What 'cellular connection' (client device) are you using to connect with?

Android. Samsung Note 20

 

[L&LD]

Android 11, or 12 on that device?

 

[JDE100]

Android 11

 

[eibgrad]

If the same client and server work over wifi, but not over cellular, that leaves only a few possibilities. Either the ISP is blocking the port (1723), or perhaps the router has a private rather than public IP on its WAN. In the former case, it wouldn't surprise me given the protocol's known security issues (my iPhone doesn't even support a PPTP client anymore, I'm surprised Android still does). In the latter case, CGNAT is a common problem. Users often don't realize they're actually configured behind their ISP's private network, which makes remote access impossible. Of course, if other services are remotely available over the WAN, that would indicate you do have a public IP.