With one Reach RS2, the accuracy of collected points will be several-meters accurate, even in PPK.
To collect precise ground control points, Reach should receive corrections. If you don’t have the second device, you can check if there is NTRIP service with reference stations within 60 km (60 km is the maximum baseline for RTK with Reach RS2). This is the easiest way to do that.
Always grab the coordinates before your flight, so you are sure the position of the GCP from the coordinate is exactly the same as is being seen by the drone.
By measuring after your flight, your risk disturbing your GCP, so doesn’t match your flight any longer.
As mentioned by the others, you need a reference to do PPK.
However, you might be able to do a kinematic PPP processing using I.e. NRCAN, and then extract the GCP’s from time stamps manually.
I have no experience with the kinematic ppp processing, only static ppp, however, I’d assume that it would be beneficial to let the receiver for a signicant amount of time before and after your GCP-measurements.