Ping from my PC (192.168.2.5) to all IPs except (192.168.10.25) works fine.

192.168.2.1 ok
192.168.0.1 ok
192.168.1.1 ok
192.168.1.25 ok
192.168.10.1 ok
192.168.10.25 not working

And when I open the Control Panel of the GL-MT300N-V2 on Site B (192.168.10.1) it shows me (192.168.10.25) as active client.

Here some screenshots:

The results of the Ping test:

IPv4 Address. . . . . . . . . . . : 192.168.2.5
Subnet Mask . . . . . . . . . . . : 255.255.255.0
Default Gateway . . . . . . . . . : 192.168.2.1

C:\Users\samfa>ping 192.168.2.1

Pinging 192.168.2.1 with 32 bytes of data:
Reply from 192.168.2.1: bytes=32 time=3ms TTL=64
Reply from 192.168.2.1: bytes=32 time=5ms TTL=64
Reply from 192.168.2.1: bytes=32 time=41ms TTL=64
Reply from 192.168.2.1: bytes=32 time=3ms TTL=64

Ping statistics for 192.168.2.1:
Packets: Sent = 4, Received = 4, Lost = 0 (0% loss),
Approximate round trip times in milli-seconds:
Minimum = 3ms, Maximum = 41ms, Average = 13ms

C:\Users\samfa>ping 192.168.0.1

Pinging 192.168.0.1 with 32 bytes of data:
Reply from 192.168.0.1: bytes=32 time=4ms TTL=63
Reply from 192.168.0.1: bytes=32 time=3ms TTL=63
Reply from 192.168.0.1: bytes=32 time<1ms TTL=63
Reply from 192.168.0.1: bytes=32 time=2ms TTL=63

Ping statistics for 192.168.0.1:
Packets: Sent = 4, Received = 4, Lost = 0 (0% loss),
Approximate round trip times in milli-seconds:
Minimum = 0ms, Maximum = 4ms, Average = 2ms

C:\Users\samfa>ping 192.168.10.1

Pinging 192.168.10.1 with 32 bytes of data:
Reply from 192.168.10.1: bytes=32 time=315ms TTL=63
Reply from 192.168.10.1: bytes=32 time=551ms TTL=63
Reply from 192.168.10.1: bytes=32 time=472ms TTL=63
Reply from 192.168.10.1: bytes=32 time=84ms TTL=63

Ping statistics for 192.168.10.1:
Packets: Sent = 4, Received = 4, Lost = 0 (0% loss),
Approximate round trip times in milli-seconds:
Minimum = 84ms, Maximum = 551ms, Average = 355ms

C:\Users\samfa>ping 192.168.1.1

Pinging 192.168.1.1 with 32 bytes of data:
Reply from 192.168.1.1: bytes=32 time=230ms TTL=62
Reply from 192.168.1.1: bytes=32 time=113ms TTL=62
Reply from 192.168.1.1: bytes=32 time=61ms TTL=62
Reply from 192.168.1.1: bytes=32 time=98ms TTL=62

Ping statistics for 192.168.1.1:
Packets: Sent = 4, Received = 4, Lost = 0 (0% loss),
Approximate round trip times in milli-seconds:
Minimum = 61ms, Maximum = 230ms, Average = 125ms

C:\Users\samfa>ping 192.168.1.25

Pinging 192.168.1.25 with 32 bytes of data:
Reply from 192.168.1.25: bytes=32 time=297ms TTL=62
Reply from 192.168.1.25: bytes=32 time=52ms TTL=62
Reply from 192.168.1.25: bytes=32 time=58ms TTL=62
Reply from 192.168.1.25: bytes=32 time=54ms TTL=62

Ping statistics for 192.168.1.25:
Packets: Sent = 4, Received = 4, Lost = 0 (0% loss),
Approximate round trip times in milli-seconds:
Minimum = 52ms, Maximum = 297ms, Average = 115ms

C:\Users\samfa>ping 192.168.10.25

Pinging 192.168.10.25 with 32 bytes of data:
Request timed out.
Request timed out.
Request timed out.
Request timed out.

Ping statistics for 192.168.10.25:
Packets: Sent = 4, Received = 0, Lost = 4 (100% loss),