Upgraded to High Sierra - Can't use 2 monitors anymore

[NealF]

Hi,

I've got a 2010 Mac Pro. Had an old samsung syncmaster 204t hooked up to it and a Dell S 2415H hooked up to the DVI port via an adapter going into the VGA port on the Dell.

All was fine in El Capitan. Now I switched to High Sierra and I can't run 2 monitors anymore.
Bought a VGA to HDMI adaptor to try going into the Dell's HDMI port. No go.

I googled it and apparently this is a problem with a lot of folks.
Any ideas on how to get these two going? Or do I just have to go back to El Capitan?

This is my graphics card, if it makes any difference. AMD Radeon HD 7950 3072 MB graphics.
Thanks.