Download and install on your PC the following:
Note: SNTP Plugin not found on later versions of NI-TimeSync than NI-TimeSync 1.2.Next, add/install the following software to the RT PXI:
- NI-TimeSync 17.1 or later
- NI-TimeSync Plugin for SNTP 1.2
- NI-TimeSync Time Reference for IEEE 1588

In order to install the NI-TimeSync Plugin for SNTP 1.2, you would need to download and install
NI-TimeSync 1.2 (as mentioned above).
At this point you should be able to configure the SNTP sync from NI MAX. Make sure that SNTP is Enabled, maybe Disable PTP (if PTP is installed too).

The last step that you would need to do is to change the NI-RT.INI and ni-timesync-sntp.ini files from the RT.
To access them, you would need to open File Explorer and search for:
ftp://0.0.0.0/, where 0.0.0.0 is the IP address of your RT PXI.
Add to the NI-RT.INI:
[TIME SYNC]
source_priority = sntp;rtc;
source.sntp.enable = True
source.sntp.address = 0.0.0.0
Where 0.0.0.0 is the NTP server IP address in this case.
Add to the ni-timesync-sntp.ini:
[TIME SYNC]
source.sntp.address= 0.0.0.0
Where 0.0.0.0 is again the NTP server IP address in this case.
Important Note: Do not change anything else in the INI files.
Additional Information
For details about adding/removing software from a real-time target, please see
Installing Software on Your Real-Time (RT) PXI Controller .