Method 1: Open the gateway panel on the mobile app, as shown below. Tap the three-dot icon and then select Device Signal Detection to get the signal strength of the sub-device.

Method 2: Search for LastHopLqi in the log and determine the signal strength based on its value.

Excellent: >200.
Good: 150 to 200.
Fair: 100 to 150.
Poor: <100

[callback-stub.c:2861] NodeId:0xab3b, LastHopRssi:-39, LastHopLqi:244
[callback-stub.c:2861] NodeId:0xab3b, LastHopRssi:-38, LastHopLqi:248
[callback-stub.c:2861] NodeId:0xab3b, LastHopRssi:-39, LastHopLqi:244
[callback-stub.c:2861] NodeId:0xab3b, LastHopRssi:-39, LastHopLqi:244
[callback-stub.c:2861] NodeId:0xab3b, LastHopRssi:-37, LastHopLqi:252
[callback-stub.c:2861] NodeId:0xab3b, LastHopRssi:-39, LastHopLqi:244
[callback-stub.c:2861] NodeId:0x17ea, LastHopRssi:-68, LastHopLqi:128
[callback-stub.c:2861] NodeId:0x17ea, LastHopRssi:-61, LastHopLqi:156
[callback-stub.c:2861] NodeId:0x17ea, LastHopRssi:-58, LastHopLqi:168
[callback-stub.c:2861] NodeId:0x17ea, LastHopRssi:-60, LastHopLqi:160