Connection via GSM LTE modem Error code 1

[gizmo_zx2]

Good afternoon.
SoftEther server (windows 10)
SoftEther VPN 4.0 (9745) client (windows 10).
Periodically client-server connection fails.
Error code 1
Connection via GSM LTE modem (megafon).
If I connect via smartphone (in modem mode) connection is fast and without problems.

Changing the port does not help.
Then after a few hours / days the connection is restored and works for a few days without problems.

What can be done?

[solo]

On the server side forward SE port and on your client disable NAT-T.

[gizmo_zx2]

On the server side forward SE port and on your client disable NAT-T.

You can learn more about it. I'm a complete beginner.

On the server side of the router I threw port 9500 and specified in Softether to listen to it.
I checked through a smartphone (in usb modem mode) to connect to this port. everything is OK.
Through GSM LTE modem (megafon) I still get Error code 1.

[solo]

Again, "on your client disable NAT-T" and tell us if in usb modem mode it connects properly.

[gizmo_zx2]

Again, "on your client disable NAT-T" and tell us if in usb modem mode it connects properly.

If I set "disable NAT-T" then through smartphone (in usb modem mode) also Error code 1. I guess something should be done on the server?
I'm sorry for the stupid questions, I'm really bad at this.

[solo]

On the server side fix port forwarding and firewall rules, ISP there may also be problematic.

[gizmo_zx2]

On the server side fix port forwarding and firewall rules, ISP there may also be problematic.

A little more information:
" disable NAT-T" = off
Connection: LAN computer - router + USB modem
I get Error code 1
Connection: computer - smartphone (wifi) - router + USB modem
Everything is ok.

I couldn't get it to work with " disable NAT-T".
router model with built-in modem = Flybox HUB71 4G+