Routing question: How to reach Media Bridge via Access Point from LAN

[Wolfschiesst]

Dear Merlin-community,

i have a setup constisting of 3 Asuswrt-Merlin driven Asus WLAN routers in my local home LAN.

LAN-Topology:

------------------- 1. floor ----------------------------------------------------
+ PC
|
| (1 GBit LAN cable)
|
+ RT-AC88U (Main Router connected to Speedport DSL-Modem; Merlin FW: 384.3_beta1, IP: 192.168.100.1,
distributing 5 GHz and 2,4 GHz WLAN, SSID: EXAMPLE# (Channel 6) and EXAMPLE_5GHz# (Channel 100)
|
| (powerline connection via 2 x Devolo DLan 1200+ adapters)
|
------------------- ground floor ----------------------------------------------
|
+ RT-AC87U (configured to run in Access Point Mode; Merlin FW: 380.69_2, IP: 192.168.100.2,
| distributing 5 GHz and 2,4 GHz WLAN, SSID: EXAMPLE# (Channel 11) and EXAMPLE_5GHz# (Channel 52))
|
| (5 GHz WLAN wireless connection; distance about 5 meters beeline, 2 wood-walls in between)
|
+ RT-AC68U (configured to run in Media Bridge Mode and connect via 5 GHz WLAN to RT-AC87U;
    Merlin FW 384.3_beta1; IP: 192.168.100.3,  link rate to AP: 1300 Mbps shown in dashboard)
-----------------------------------------------------------------------------------

Problem description:
I cannot reach the webinterface of the RT-AC68U Media Bridge on IP 192.168.100.3 as long as it is connected to the 5 GHz WLAN of the Access Point RT-AC87U from my PC using a browser or using ping.
Browser states: Network timeout.

When I turn of the RT-AC87U, it connects to the 5 GHz WLAN of the RT-AC88U and everything works fine as intended. Then I can reach the webinterface from PC, successfully.

I now tried to setup the followingmanual route on the main router (RT-AC88U) as a solution:

Network       | Netmask       | Gateway       | Metric | Interface
192.168.100.3 | 255.255.255.0 | 192.168.100.2 | 1      |LAN

But still it is not working, as long the RT-AC68U is connected to the RT-AC87U.

Does anyone have an idea for a solution?