Info as provided by Draytek:

Before Creating Mesh Network:

1. Survey your environment and your floor plan. Use the Wired link as possible to get the best performance if connecting Ethernet between devices is possible.

2. Using Vigor Router as Mesh Root:
Vigor2862/ Vigor2926 WiFi router can be connected to AP903 and AP802 mesh nodes but does not support AP912C or newer AP. This support list will be added to WebUI of the router in the future firmware version.
The tracking wish G-ID is [G81899] [2862][2926][2927] Adding Mesh Model support list in the Web UI   

     To let Vigor Router work with AP912C

  •      Vigor2862/ Mesh Router----(Wired link)----AP912(Mesh Root)-----Mesh Network


3. Make sure the signal strength is strong enough, we can detect it by using AP Discovery function on both devices.

4. Mesh network is based on WDS, the Wireless channels between two Wireless Mesh devices must have some same 5G channels. Otherwise Mesh network cannot be connected.

For example:

AP918R is an outdoor AP. It only has 5G band3 for EU countries.

However, AP903 only supports Mesh on 5G and its channel is set as channel 40. As a result, AP918R cannot create a WDS connection with AP903.

 When get trouble to connect mesh network, please

1. Enable Detailed as Log Level and collect Mesh Syslog to us.

The new web page to show mesh log will be supported in the future firmware.

The tracking wish G-ID is [G82325] [AP912/Mesh models] Change Mesh Log Level to "Detailed" by default

After enabling "detailed" log level, you can see the connection logs with [dmn] prefix as shown below.

‑141­‑2019-02-27 16:14:40­‑Feb 27 16:14:38­‑AP903­‑[dmn] Add List is empty! Still start to register.­
‑141­‑2019-02-27 16:14:40­‑Feb 27 16:14:38­‑AP903­‑[dmn] dmn_pkt_send Announce-GoRegister­
‑141­‑2019-02-27 16:14:40­‑Feb 27 16:14:38­‑AP903­‑[dmn] Mesh generate my public key successfully­
 ‑141­‑2019-02-27 16:14:40­‑Feb 27 16:14:38­‑AP903­‑[dmn] Register start.­

2. Getting issues when using Router as Mesh Root and Mesh logs shows following messages.

2020-06-10 07:38:34 [dmn] 00:1D:AA:EE:27:F5 RSSI is too low (16/30) 18 sec
 2020-06-10 07:38:33 [dmn] 00:1D:AA:EE:27:F5 RSSI is too low (16/30) 17 sec

(16/30) : Mesh Root will check the signal strength of the Mesh Nodes, 16 means the signal strength of mesh node, and 30 is the RSSI threshold. Mesh network cannot be connected successfully when signal strength of Mesh Node is lower than the threshold .

We can prevent the issue happens if we set RSSI threshold to be lower.

Telnet command to set up RSSI threshold

DrayTek> dmn set ?

% Usage : dmn set <action> <value>

% <action> :

% rssi - set rssi threshold (30 for default)

% timeout - set timeout level (in second)

If problem persists, please try to use the topology in Before Creating Mesh Network (2)

3. Set up Range Extender by 2.4Ghz(Universal repeater mode) for workaround.