Modern routers and access points have the possibility to transmit on 2.4 GHz, 5 GHz or on both frequencies.
In general, with a 5 GHz network, you will always get better reception because the band is less used. Most other wireless devices transmit in the 2,4 GHz band, thus it is mostly overcrowded.
IMPORTANT: The 2.4 GHz network always has a greater range than a 5 GHz network.
The default setting is dual mode, i. e. both 2.4 GHz and 5 GHz. In this case, Apple devices automatically "choose" the mode with the better reception.
A maximum of 13 channels are available for a 2.4 GHz WiFi in Germany. However, since these overlap, it is best to use channels 1,6,11 and 13.
Up to 19 non-overlapping channels are possible in a 5 GHz network.