Yes, a repeater must be on the same channel as a gateway within range to mesh properly.
While you should use the Auto Channel feature to ensure each AP is on the best channel, you can manually set access point channels in Datto.
First make sure that the channel setting under Configure > Radio is set to manual. Then under Manage > Access Points, click the gear icon next to an AP and and select Edit. From there you can change the channel override settings.