I bought an 8GB stick off amazon for $36, but mine is a GL63 8SC if that matters. Worked perfectly. https://www.amazon.com/gp/product/B07JWG53DG

I'm getting the same unit and didn't know about ram but found this after I saw your question, he may answer and have a better solution. https://mariushosting.com/synology-which-ram-to-buy-for-ds920-nas/ (not my site)

Edit: found this about getting to 20GB https://www.reddit.com/r/synology/comments/p670yd/ram_upgrade_works_with_ds_920_now_20_gb_ram/

and this 8GB on sale on amazon for $29 normally $49 if you just want to get to 12GB that was tested as working to 12GB (4GB onboard that can't be removed) https://www.amazon.com/Samsung-2666MHz-Memory-Laptops-M471A1K43CB1/dp/B07JWG53DG (while these are not on sale, they are $55 normally on amazon https://www.amazon.com/Crucial-16GB-Laptop-Memory-CT16G4SFRA32A/dp/B08C511GQH) and cheaper on Newegg https://www.newegg.com/crucial-16gb-260-pin-ddr4-so-dimm/p/N82E16820156262 and slightly on sale also for $37