@niazstinu if you have synced users you must have an Exchange console onprem for attributes editing
"When directory synchronization is enabled for a tenant and a user is synchronized from on-premises, most of the attributes cannot be managed from Exchange Online and must be managed from on-premises. This is not due to the hybrid configuration, but it occurs because of directory synchronization. In addition, even if you have directory synchronization in place without running the Hybrid Configuration Wizard, you still cannot manage most of the recipient tasks from the cloud. For more information, see this blog article."
regarding dns you can find correct settings in your tenant admin center under domains