ActiveSync issue exchange 2016 on-prem

Hello all,

    I have 3 problem hope support me to resolve it.

1- Can't add new email on some mobile devices "iPhone and Android" after add email, password, server and domain showing error "unable to support"

NB.: Not for all users also not for new email, sometime old email log out and can't login again

App used on mobile: Outlook
Connection: ActiveSync
Exchange 2016 CU 18

2- Not receive any notifications on the mobile, knowing the notifications are allowed from the settings. (All users).

3-Have shared mailbox the user in shared mailbox have (Full Access and Send As) but can't send to more than 15 recipients, knowing I check (mailbox features ---> Mail Flow ---> Recipient limit) is unlimited.

Thanks for your support