VPN Connection Issue

Hi all,

I'm wandering around a client-to-site VPN access issue.
Scenario: HQ USG310 (will be be replaced with 500H) with IKE2 with cert. client-to-site VPN and about 40 remote clients.
Some of them have strange connection issues "seemingly" related to the internet connection.
I took a couple of laptops and phones from the customer site to do some lab tests and now on my desk i have:
* Laptop1 (win11)
* Laptop2 (win10)
* Mobile Phone1
* Mobile Phone2 (they use the same mobile operator and same configuration APN,etc..)
* Lab's internet connection.

The customer complains about errors during vpn connection in certain conditions. Below are some tests that I have done in the lab and at the customer's home.

In any condition, ports on USG are reachable and open

and this is what I see on the usg log

From the logs it seems evident that when the connection is not established, all traffic on the 4500 is absent. I wondered if it was the mobile operator or the device that was blocking this traffic, but the answer is no. In fact, the other laptop connected to the same mobile phone (at the same time) has no problem.

I was forgetting: I use the native windows vpn client and the error is: "Unable to establish connection between the computer and the VPN server. The remote server is not responding...."

Any suggestions are welcome.

Thanks in advance

Lorenzo