Mellanox CX312A Low Profile Card in a RS3621RPxs

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It should work. You can try, there is nothing you can break.

Here is an official list

Synology E10G18-T1 1 x 10GbE RJ45 PCIe 3.0 x4
Synology E10G18-T2 2 x 10GbE RJ45 PCIe 3.0 x8
Synology E10G21-F2 2 x 10GbE SFP+ PCIe 3.0 x8
Synology E10M20-T1 1 x 10GbE RJ45 PCIe 3.0 x8
Synology E25G21-F2 2 x 25GbE SFP28 PCIe 3.0 x8
Emulex OneConnect OCe11102-FM 2 x 10GbE LC Connector PCIe 2.0 x8
Intel Ethernet Converged Network Adapter X520-SR2 E10G42BFSR 2 x 10GbE LC Connector PCIe 2.0 x8
Intel Ethernet Converged Network Adapter X710-T4 X710-T4 5.05 4 x 10GbE RJ45 PCIe 3.0 x8
Intel Ethernet Server Adapter I350-T4 I350T4 / I350T4BLK 4 x 1GbE RJ45 PCIe 2.0 x4
Intel Gigabit ET Dual Port Server Adapter E1G42ET / E1G42ETBLK 2 x 1GbE RJ45 PCIe 2.0 x4
Intel Gigabit ET2 Quad Port Server Adapter E1G44ET2 / E1G44ET2BLK 4 x 1GbE RJ45 PCIe 2.0 x4
Mellanox ConnectX-4 Lx EN MCX4121A-XCAT 14.20.1010 2 x 10GbE SFP28 PCIe 3.0 x8
Synology (EOL) E10G17-F2 2 x 10GbE SFP+ PCIe 3.0 x8