Sorry for causing the misunderstanding! Let me correct myself.
RTKLib shows different observation codes. The observation code refers to the frequency according to the RINEX format specification. So, the L2C code of observation in RINEX roughly equals the G2 frequency of GLONASS, as it’s shown in the screenshot I’ve posted in the previous post.
As Christian has correctly mentioned, L2C frequency is to be provided by the GPS constellation. According to the RINEX specification, the observation code for this frequency is L2X.
According to Arnaud’s log, we can be sure that the Reach RS2 has recorded the L2C frequency. The correct screenshots to prove this are below: the first one shows that the L2 frequencies have been recorded and the second one shows that the L2C frequency has been recorded.