It's not normally recommended to add teachers to the site because you may have hundreds of classes, they'd get notifications from all of them and the courses won't appear in the dashboard - but if you only have a small site and a few courses then follow the instructions in the FAQ Why can't I add teachers and students site wide in Moodle?, using the non-editing teacher role instead of the editing teacher role.
Teaching with Moodle
Moodle 3.6 - Assigning the teacher role
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