I think you're confusing the Roth IRA with the new Roth TSP. The TSP, like a 401k plan, has a $17,500 limit on total contributions between the regular TSP (tax deferred) and the new Roth TSP (pay taxes up front). Separately from that, you can put $5,500 into your Roth IRA with after-tax money at some place like Vanguard and your employer is not involved in any way.
For most people here, the best advice is to max the $17,500 in the traditional TSP to get the up front tax deduction (and full match) and then begrudgingly put $5,500 into your Roth IRA because it's better than investing in a taxable account.
The Roth TSP doesn't make a lot of sense for an someone trying to retire early unless you have some unusual circumstances, like combat pay.