You've probably already figured this out, but investing in U.S. securities without a Social Security Number is tricky and complicated. Sometimes it's impossible.
My suggestion is to find a certified financial planner (CFP) to speak with. Try Googling "CFP non US citizen clients" or "CFP for foreign investor." Something like that.
A CFP can listen to your situation and recommend specific online brokerage accounts you might be able to use to purchase bonds, gold/silver ETFs, etc. They can also explain tax implications and other important details.
CFPs are required to only make recommendations in your best interest. Speaking with this type of professional is the best way to get objective, honest and customized financial advice.
The hourly rate for CFPs can be rather high — usually $100-$250. But nearly all of them offer free consultations. Just explain your situation during the free consultation and see if they're experienced enough in that area to help you. From there, you can decide if you want to pay for an hour of their time.