I was able to do a search and according to the manufacturer Synology, the model DS1525+ has 2x SO-DIMM slots and the following model is the one recommended by the manufacturer:
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Synology D4ES03-16G (1 x 16GB, 2666 MHz, DDR4-RAM, SO-DIMM)Synology's recommendation is based on their tests and data. However, numerous components from third-party manufacturers have already been used, which is correct. As the requested RAM modules are even slightly faster, they can also be used at a slower speed, so they are technically compatible.
Alternatively, these are also available here with 2666mhz:
Corsair Vengeance (2 x 16GB, 2666 MHz, DDR4-RAM, SO-DIMM)