Does the distance from the Sagemcom f@st 3864 have any affect on the problem?
Have you tried changing the wireless channel?
Hi - Has this recently occurred after stability of connections for a long time or have you just connected to iHeart radio and found disconnections. If all your other Internet connections from your mobile phone using the modem's 5Ghz SSID are stable, I don't think the change of band selection would make a difference specially when you are in close proximity to the modem. Although it is a weaker signal at an extended range or handicapped by many walls, when that happens the speed is reduced until it is compromised and in those circumstances, your mobile should swing you over to the stronger 2.4Ghz frequency anyway. I am not familiar with iHeart, can you listen to it from a Wi-Fi enabled laptop or tablet to see if it's 5Ghz Wi-Fi Adaptor is stable?
There is a inconvenience factor if you have to swap bands manually on your mobile for stability, but you could try selecting the mobile's 2.4Ghz setting as an experiment to see if things improve - depends how often you use the mobile for accessing the Internet when home. I tend to use my PC rather than my mobile. May pay to check if the App has any updates in case it improves stability.