cristinaslavita Welcome to the community!
I have adjusted the 2.4GHz WIFI on your modem and changed the name to UPC277xxxx_2.4GHz. Since you have 3 devices connected to the 5GHz WIFI, I recommend you to connect the devices which are further away from the modem to the UPC277xxxx_2.4GHz WIFI. The devices that are closer to the modem can be connected to the UPC277xxxx WIFI.
I have also changed your modem to IPv4. The conversion is done in 30 minutes. After that, restart your modem. If you have difficulties, perform a modem reset. Press the reset button on the back of the modem gently with a pointed object for at least 25 seconds.
After that, please let us know if your reported error still exists.