Hello,
When devices are in TAP mode, they should reach each other without any additional configuration.
Can you check if the devices have different LAN IP addresses, let’s say the server, router D, LAN IP is 192.168.2.1 and client, router A, LAN IP is 192.168.2.2, if the clients have the same IP as the server it won’t work.
If it is set up like that, could you then attach troubleshoot files from both devices A and D, you can do that via WebUI System -> Administration -> Troubleshoot and then press download in the troubleshoot tab. You can find the detailed instructions in our wiki.
Also, could you update your topology with WAN and LAN IP addresses and network masks.
Regards,
Paulius