https://semiconductor.samsung.com/dram/ddr/ddr4/
The router has 2 k4a4g165wf-bctd chips, 512 MB each (256M x 16). To increase it, you probably need 512M x 16 or 1G x 16 or 2G x 16. Theoretically, 8GB can be supplied. I looked at the sources. It’s not immediately clear what code is responsible...