The following steps can be used to add your domain email address(es) for use with the Gmail app.
1) Open the Gmail app
2) Tap your profile icon (top-right)
3) Select Add another account
4) Choose Other
5) Enter your full email address (e.g. mailbox@domain.com)
6) Tap Next
7) Enter your mailbox password
Incoming mail (IMAP) settings:
Server: imap.extendcp.co.uk
Port: 993
Security: SSL/TLS
Outgoing mail (SMTP) settings:
Server: mta.extendcp.co.uk
Port: 587
Security: STARTTLS
Username: Full email address
Password: Mailbox password