Suddenly RSSI and SNR decreased of nodes


We have installed about 300 nodes, and 4 gateways in 3km of area, and running from 20 days.
All nodes data comes from 18 days. Suddenly from last 2 days, We are getting data of only 20-25 devices.
And also these 20-25 devices RSSI and SNR decreased by sharply, can say from +8 SNR to -10 SNR, in all 20-25 devices.
We also checked that Node and Gateway, Antenna, is connected properly, and doesn’t seems to have any issue,
and we doesn’t change any software or hardware side configuration in 20 days.

Please guide us what can be issue in this case?
Hs anybody face this type of issue?

Thank You

By RTL-SDR module analysis:

Can it impact on 865 to 867 mhz?