The item ID is B08KRXFM4R since the link doesn't seem to be rendering in my reply.

I can't vouch for it working on the DS1621xs+ of course, but the RAM specs do seem compatible. The DS1621+ specifically requires dual rank RAM, which is why I bolded it.

The Xeon D-1527 supports up to 128 GB RAM. Just throwing that out there ;)

The DS1621+ is using the AMD Ryzen 5 1500B embedded processor that's soldered to the motherboard. AMD itself states that this processor only supports 32GB of memory. Synology's specifications logically state that the NAS supports a maximum of 32GB of RAM. When I purchased the NAS I also purchased a 32GB RAM upgrade to go with it: TimeTec sells 16 GB SODIMMs that they market as a replacement for Synology's official D4ECSO-2666-16G part. That worked well and starting off fully upgraded made for a great user experience.

NASCompares has a page where they discuss the ability of doing a 64 GB RAM upgrade on that NAS / CPU. This is much more of a risky proposition considering AMD itself states their processor doesn't go over 32GB. So 3 weeks ago I decided to give it a go. I was going to go with one of the Kingston modules recommended by NASCompares but ended up sticking with TimeTec RAM since it was readily available and met the correct hardware specs.

Timetec 64GB KIT(2x32GB) DDR4 3200MHz PC4-25600 Non-ECC Unbuffered 1.2V CL22 2Rx8 Dual Rank 260 Pin SODIMM.

I wish I could add more drama to the story, but I just popped the old RAM out from the trapdoor at the bottom of the unit and popped the new RAM in. The NAS POSTed and booted with no complaints from the BIOS about incompatible RAM. The DSM system information screen and resource monitor both report a full 64 GB of RAM! My NAS has now been running with 64GB for 3 weeks and I'm beyond pleased! I now have more than enough capacity to add the core services to my NAS that will support the more experimental things I'm doing with my actual computing and virtualization lab.