Sql Server 2016 uses low RAM
Like @Shanky mentioned in the comment, it could be due to Express Edition accidently being installed instead of Standard or Enterprise. Due to the Database size size limit, I don't think you could even restore a large database, so you would notice almost immediately. 

As an experiement, have you tried caching a lot of data in memory by doing several large SELECTs to see if it changes? SQL Server is fairly memory-hungry, but by default it doesn't use memory until it needs it, so if nothing has happened since the last restart, there's nothing in memory yet.