Connect Apple iMac 2017 27" (Thunderbolt 2) to 30-inch Apple Cinema Display HD

The Apple Mini DisplayPort to Dual-Link DVI MUST have 5 volts on its USB pigtail or the adapter can't work. Anything that can provide that 5 Volts will do. If you also want the brightness slider to work, you need USB power AND data over that USB.

Watch out for adapters that ship direct from the Far East -- they really will take the rated three weeks to arrive.

Thinking if I added the Thunderbolt 2 to 3 adapter it would work, but it won't power the monitor

It won't work anyway. That ThunderBolt-3 to 2 adapter has a specific disclaimer that it does not provide a Usable DisplayPort or any other display signal (it is encapsulated and devices can't read it, except genuine ThunderBolt devices).

You need a third-party USB-C adapter to Mini DisplayPort (preferably rated for 4K -- by which vendors mean 3K), then plug in the Apple Dual-Link DVI you were using, then get an iPhone charger to give you the 5 Volts you need over USB to power the Apple adapter. That gets you everything except the brightness slider, which requires a real USB connection, not just an iPhone changer. But how often do you need to change the brightness.

just sold mine on eBay. Go to bay. New they sold for $100.00 dual dvi is what you need.