RX quality differs significantly between devices: RSSI / SNR

I am having troubles to understand the following:

I am installing two devices (Dragino LT-33222-L with an external antenna and Elvaco CMi4110) in a building located around 500m from the next Gateway (Mikrotik WAP LR8 Kit). It is not line of sight, but there are just a few walls in between.

When using an adeunis network tester right next to both devices the RX strength is as I would expect:

  • SF: 8
  • RSSI: ~ -110
  • SNR: ~ 0

However, both above mentioned devices are received significantly worse by the gateway:

  • SF: 10-12
  • RSSI: ~ -110
  • SNR: -15 - -20

While the RSSI is roughly the same, the SNR is significantly worse.

What could be the issue here? Any idea?