@TP-Link Thanks for the quick reply. It is important to tell that I upgraded firmware to 1.6 and tried both Router and AP mode. Also, the 2ndM5 is located a little far away (about 60ft) from MainM5...

Got a 3 Deco M5 system and are currently using 2. Have the main unit, connected to a switch, which connects via Ethernet wire to ISP ONT (modem) Huawei HG8145V5 on DHCP. The second unit, connected...