Cannot access web server using Wifi repeater

I have the following configuration:

1. Router A: main router that receives internet.
2. Router B: Wifi repeater of router A
3. Server C: a server connected to B.

When I connect to B, server C can be reached when no problems. Question: why when I am connected to A, I can no longer access B.

Thoughts: it seems like that the repeater is making its own LAN that is isolated from LAN of router A. A little explanation of what is happening would help.

Background: Router: Huawei E5172s-920, Repeater: TL-WR740N.

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