What should I do after receiving a 6-month multiple entry tourist visa instead of a DTV?

something in your application to the DTV did not appeal to the processing embassy, something caused them to decide that you were not elegible to the DTV. So instead of cancelling your application and keeping the application fee (which is not refundable), they at least issued a 6-month multi entry Tourist Visa to minimize the loss

Can you read the visa and Google ? It mean exactly that

Workcation or soft power?

A dtv is a tourist visa...what's the expiration date on the visa?

After 59 days

You need to go to

Immigration and extend for

30 more days

After 89 days you need to leave Thailand and come back in to restart the Multi Entry Evisa

It means what it says. It is a visa that is valid for 6 months and every time you enter Thailand during that period you get stamped in for 60 days. The consulate obviously judged that was sufficient for your requirements but without knowing on what basis you applied and if it was soft power how long a course you signed up etc there's no point guessing. You can't apply for another visa until that one expires as you can't simultaneously hold 2 visas for Thailand