I ordered an RS485-Wifi bridge “Elfin EW11A” (aliexpress, cost around 8€) , with external antenna and connected it today to the inverter (using RS485A1/B1). It works flawlessly up to now (few hours). I used the inverter USB dongle connector as the power source so that everything remains simple. This makes a very cheap, and apparently very reliable alternative to the Huawei wifi dongle.
After connecting the device to the inverter RS485/USB power, I just connected to the access point the EW11 presents by default. From the EW11 web interface, I just put the device in STA mode, connecting it to my wifi router (entered SSID and password), I set the serial baud-rate to 9600, set the serial protocol to modbus. And this is it. The default TCP port to use is 8899 and huawei solar just likes it (slave id 1).