It turns out that I have already tested all possible channels (5300-5500) and none gave a good signal.Nothing about interference. Your picture clearly states too weak signal at -80 dB. You need to reach around -55 dB to get good link.
You can help me with this!?Create VLAN/another L2 segment between RB3011 and CPE
RB3011(start PPPoE server here)------ether/vlan------RB433ah(bridge ether/vlan+wlan1/2/3)---------------PPPoE Client CPE