RN528 - how to determine RAM config

You don't need sudo, since you are logging in as root.

You can install dmidecode if you like with 

apt-get install dmidecode

My RN526x has one DIMM module installed.

root@NAS:~# dmidecode --type 17
# dmidecode 2.12
SMBIOS 3.0 present.
# SMBIOS implementations newer than version 2.8 are not
# fully supported by this version of dmidecode.

Handle 0x005B, DMI type 17, 40 bytes
Memory Device
        Array Handle: 0x0059
        Error Information Handle: Not Provided
        Total Width: 72 bits
        Data Width: 72 bits
        Size: 4096 MB
        Form Factor: DIMM
        Set: None
        Locator: DIMM_A1
        Bank Locator: NODE 1
        Type: Other
        Type Detail: Synchronous
        Speed: 2133 MHz
        Manufacturer: Undefined
        Serial Number: 0013EAFE
        Asset Tag: DIMM_A1_AssetTag
        Part Number: TS512MLH72V1H
        Rank: 1
        Configured Clock Speed: 1866 MHz
        Minimum voltage:  Unknown
        Maximum voltage:  Unknown
        Configured voltage:  0.003 V
...

Googling the part number indicates this is a Transcend part - DDR4  ECC Long-DIMM 4 GB