This is just a quick note to save anyone else time that might hit this. The Synology DS Assistant tool (the one you used to install the DSM on your new NAS) also has a memory test function. You need to enable this within Preferences of the tool. Once you do that you can have it reboot your NAS and perform a memory test. This is useful if you have a new NAS or upgraded the memory in an existing unit. It’s something I highly suggest you use.
What I ran in to was that I use LACP (Link Aggregation Control Protocol) LAGs (Link Aggregation Groups) with one of my Synology boxes. These let me group multiple NICs together for better throughput. When the DS Assistant reboots the NAS to perform a memory test it doesn’t use your configured network setup. It uses DHCP to grab an IP and just does a basic configuration (no LACP, no VLAN tagging, etc). Therefore when it would reboot my switch couldn’t talk to it via LACP and none of the ports came up…and the DS Assistant wouldn’t see the NAS anymore and wouldn’t report status of the memory check.
So before you do this I advice breaking your LAGs or using an extra interface (if you have a spare one on the NAS) and connect that to your switch on a normal access port. Just a heads up.