The 510 doesn't do TBT. The only 'turn guidance' you'll get on the 510 is if, say, there's a sharp change of direction in the course ahead, it will tell you to 'Turn Left' regardless of whether it's an actual junction or just a bend in the road.
If you had true TBT instructions on your audax, they were most likely embedded in the GPX/TCX file as custom cues. I believe you can add turn instructions to a course in RWGPS, but you can only export the file with those instructions included if you have a premium account. I've never bothered with premium membership of RWGPS so I don't really know how it works.