Hi @Pham Tien Dung,
Welcome to the Microsoft Technical Support Forum.
According to your description, you want to import certificates in ECP, but you don't find the more option "..." button. I would suggest you to try the following format command in Exchange Management Shell to import the certificate:
Import-ExchangeCertificate -FileData ([System.IO.File]::ReadAllBytes('<FilePathOrUNCPath>')) [-Password (Read-Host "Enter password" -AsSecureString)] [-PrivateKeyExportable <$true | $false>] [-Server <ServerIdentity>]
Refer to: Import or install a certificate on an Exchange server | Microsoft Learn
Please feel free to contact me if you have any queries.
Best,
Jake Zhang