This was most likely related to Buffalo AP. With TP-Link Acher Raspberry seems to work just fine.

Meanwhile I designed for the Raspberry 3B and relay module:

Reported speed 72Mbps. That is WiFi N. Thank you for the information. The router shows: MAC Address Lease IP Address Hostname Communication Method Wireless Authentication 802.11n B8:27:EB:xx:xx:xx - - Wireless Authorized Disabled For other devices, such as ESP32-Wroom-32, 802.11n is Enabled. Perhap...

I have strange issues with Raspberry 3B WiFi. What I see is that connections are lost frequently and they come back if I ping from Raspberry to address I want to connect. As an example, if my ESP32 is connecting to Raspberry with mosquito mqtt-service once a minute, everything work just fine if I l...