Typically I'd expect the R740XD to 'see' the H810 just fine, except maybe in OMSA and/or the iDRAC (other than recognizing it in a PCIe slot). However, if your R740XD only came with a single processor, not all PCIe slots are functional. This could easily be a reason why the card isn't recognized. This page has an overview of which PCIe slots in which riser attach to which processor. If you do have just 1 processor, to upgrade to dual processor (so you can use all PCIe slots), will require an exactly matching processor. If you already have dual (identical) processors, this obviously wouldn't be a possible cause to your issue. Another option is to add a PERC H830 to your server, get a 12Gbit to 6Gbit SAS cable and connect the MD1220 to the PERC H830.
We purchased an R740xd a few months back and were told that it would be no problem attaching to a MD1220. The 1220 is loaded with 24x1.6TB MU SSD drives at large cost so there is no easy way to just replace it. However, it now appears that is not (easily) possible. So I had a few ideas and was hoping someone else had at least attempted them: 1 - Buy a 1420 with minimal drives, put in the 1.6TB SSD drives from the previous (MD1220) array. I belive they would need new drive caddies, but I am hoping at least the drives themselves would work 2 - Instead of a Dell PERC adapter, use a 3rd party adapter like Adaptec or LSI. My concern here is that the R740xd doesn't even see the older PERC adapters like the 810 so I am not sure if it would see these either. Any suggestions would be helpful
PowerEdge R740xd to MD1220