You can install SO-DIMM DDR4 3200 ECC modules, Timing 22-22-22 up to max. 32 GB.
Note that these are ECC modules, so not the ones used in current laptops! (the bytes are 9 bits, with one error control bit).
You can easily find them on this site.

I bought the following module: "Kingston KSM26SED8/16HD" part number: "13953675"" and have had no problems so far, except when I reboot, I get a warning that the module is not officially compatible. In the event of a warranty claim, you just have to rebuild everything.

Synology maintains a product list with approval for this...