If you are trying to connect your device and you are not able to, the problem is probably that your smartphone or tablet is connected to a 5GHz network.
If you can't check which network your smartphone or tablet is connected to, below are two manuals from different operators with which you can disable this network momentarily while synchronizing, once your device is synchronized you will only have to activate it again and you can control your device, connected to the 2.4GHz network, from your smartphone or tablet connected via the 5GHz network.
You can check the alternative synchronization modes in this article.