At DDR (Double data rate) that means they are actually at 2133mhz, which is the default speed for most DDR4 memory.
Second and fourth slots, is right, but make sure that it is the second and fourth slots counting to the right, starting at the CPU socket. Meaning, the 4th slot will be the one closest to the edge of the motherboard, so if it IS, then it is correct.
The first thing you should do is make sure you have the latest motherboard BIOS version installed because memory compatibility is one of the primary focuses so far, as usual, with X570 motherboards.
After updating, if you are not already on the latest BIOS version available for your board, it would be advisable to THEN do a hard reset of the BIOS by removing the CMOS battery for five minutes, during which time you should press the case power on button for 30 seconds continuously. All of which should be done with the power disconnected from the wall first. Then reinstall the CMOS battery. Reconnect power. Power on and go into the BIOS and select the XMP profile in the memory settings. Save settings and exit BIOS.
If it will POST, great. If it will not, then remove the CMOS battery again to rest the settings (Or use the BIOS reset button or jumper) and start over. Go into BIOS, set the XMP profile and this time increase the memory (DRAM) voltage by .005-.020v. Save settings and exit BIOS. See if it will POST again.