I have the same NAS. There's nothing really magical about the ram. It's just average DDR4 SODIMM ram. Here's an 8gb x 2 pair for 32 bucks I installed in mine .

https://www.amazon.com/gp/product/B0B8NTKC19/ref=ppx_yo_dt_b_asin_title_o02_s00?ie=UTF8&psc=1