ram usage

so i just got a dell latitude 15-3521 from a guy with "35 yrs" exp. working on then and swears he never took off the back cover which was missing some screws and others were loose. this is the problem he says it had a hard drive issue so he replaced a 500gb with a 250gb which doesn't matter but even with a clean install of windows 10 pro i show only 256mb usable ram from 8gb the same if i change to 4gb and 2gb also have tried msconfig chking and unchking memory in boot section. please tell me something other than i bought one needing a motherboard---thanks