I am in trouble because I do not know the cause of the notation. Would you please teach me?

I am in trouble because I do not know the cause of the notation. Would you please teach me?
◆ Environment
・ Terminal side
OS: android10
Email software: outlook, Gmail, emailwise
-Server side
Exchange 2016 (on-premise)
·network
The server is located on the internal network
The terminal is docomo and connects with a closed network called Access Premium
◆ Event
-Connected with both Activesync and IMAP, but NG status
・ Email can be used with Activesync in Mailwise
・ Although it can be set in Gmail, the folder is displayed only halfway.
When I searched, the reception list was displayed for some reason, but I could not send an email. (Both Activesync and IMAP)
-In Outlook, a timeout error occurs when setting an account. (Both Activesync and IMAP)
From the server's point of view, it seems that the connection is not even made and there is no log.
・ In Gmail, we received a report from a user that mail can be used when connecting with an android8 terminal.
(The above phenomenon when using android10)
◆ Question
・ In the first place, Exchange 2016 (on-premise, closed network) and smartphone apps with Gmail app and outlook app
Do you have a connection record?
・ If you have information about the same unreachable event in the past, could you please tell me what to do?
◆ Past survey (reference)
So far, I have asked some questions at Microsoft's paid window, but
The cause was not found from the log on the Exchange server, and there was no record of on-premise or closed network, so I could not answer any more.
The following is an excerpt of the survey results.
-It has been confirmed that the settings are the same for the current Exchange server and the new exchange server.
· Create a new mobile device mailbox policy (password digits, password complexity, etc.)
I assigned it to a verification email account, but the email was still available and the situation did not change.
・ I explicitly set connection permission for the new KSK smartphone mobile, but the situation did not change.
* In the first place, all mobiles are allowed to connect by default.
・ The situation did not change even if the certificate was installed on the mobile device.
-I got the server log and asked the MS paid window to investigate, but the cause is unknown.
・ I investigated from the viewpoint of MDM, but the cause is unknown.
-There is no record of changing the algorithm of the server certificate.