UPDATE: I followed your instructions and loaded the route just fine.
While I was using it for navigation, sometimes it would give me useful instructions like "turn right", "turn left", but other times pretty meaningless instructions like "go northwest". Is this your experience too?
The Edge 510 can't plot routes. It can give a rough direction to a waypoint eg "go northwest", that's just the limitation of the hardware.
However, your route file can contain directions. These are simple messages that are triggered in close proximity to a set of coords; (you can see the raw data from a GPX extract below). It's not real routing, but it works as long as you stay on the route.
The other problem with the 510 is the lack of maps. These can be added but the 510 isn't really up to the job. Best avoided. There's always the breadcrumb trail:
GPX extract: ("navigation messages" are in bold)
<?xml version="1.0" encoding="UTF-8" standalone="no" ?>
<gpx xmlns="
http://www.topografix.com/GPX/1/1" creator="
www.plotaroute.com" version="1.1" xmlns:xsi="
http://www.w3.org/2001/XMLSchema-instance" xsi:schemaLocation="
http://www.topografix.com/GPX/1/1 http://www.topografix.com/GPX/1/1/gpx.xsd">
<metadata>
<desc>Route created on plotaroute.com</desc>
</metadata>
<rte>
<name>Test</name>
<rtept lat="51.370198" lon="-1.12107">
<ele>102</ele>
<time>2024-01-20T00:00:00Z</time>
<name>Start on R</name>
<cmt>Start on Reading Road</cmt>
<desc>
Start on Reading Road</desc>
<sym>Flag, Blue</sym>
</rtept>
<rtept lat="51.370209" lon="-1.121036">
<ele>102</ele>
<time>2024-01-20T00:00:02Z</time>
<name>At roundab</name>
<cmt>At roundabout, take exit 4 onto Reading Road</cmt>
<desc>
At roundabout, take exit 4 onto Reading Road</desc>
<sym>Flag, Blue</sym>
</rtept>
<rtept lat="51.369905" lon="-1.121671">
<ele>101</ele>
<time>2024-01-20T00:01:54Z</time>
<name>Turn right</name>
<cmt>Turn right onto Red Lane</cmt>
<desc>
Turn right onto Red Lane</desc>
<sym>Flag, Blue</sym>
</rtept>
<rtept lat="51.37158" lon="-1.122592">
etc etc