The answer
An address such as office@yourname.at needs a mailbox with an email service. Domain registration alone does not create that mailbox. atdomain.at currently does not sell email mailboxes.
Your email service supplies the required MX, SPF, DKIM and DMARC values. Copy the complete values and avoid multiple competing SPF records for the same name.
What to do
Set up the domain and mailbox with the email service.
Add the supplied records to the active DNS zone.
Keep existing website records.
Test sending and receiving with another mailbox, and check spam folders.