wifi does not work on 5Ghz with newer kernel - ath9k

[v7peer]

I have a Atheros PCI wifi adapter with chipset AR93xx. De driver is ath9k.

In kernel 4.15.0-36-generic I have support for 2.4Ghz and 5 Ghz.
In kernel 4.18.0.10-generic (or newer) I only have support fot 2.4Ghz.

How can I enable the (lost) 5Ghz support in the newer kernel