Hi Julian,
Thanks for posting your concern.
In order to change ad rotation setting of ad group by using the API, you need to clear the adGroupAdRotationMode setting altogether. This means sending an AdGroup SET operation that looks like this in SOAP:
<operator>SET</operator>
<operand>
<id>12345</id>
<campaignId>567890</campaignId>
<campaignName>Sample campaign name</campaignName>
<name>Sample ad group name</name>
<status>ENABLED</status>
<adGroupAdRotationMode>null<adGroupAdRotationMode/>
</operand>
Also, depending on the client library, you can effectively do this by setting adRotationMode field to null. For example, in Java this looks like this:
rotationMode.setAdRotationMode(null);
Let me know if you have further questions.
Regards,
Ejay
Google Ads API Team

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