This message was posted on behalf of Intel Corporation
Thanks for posting in our communities.
We understand that you're trying to prevent Debian 9* from connecting to 5GHz networks in order to use it as an access point (AP). As you've noticed, the adapter can only work as an AP on 2.4GHz, and it can only work with one wireless band frequency at a time (either 2.4GHz or 5GHZ).
On Windows*, there's a wireless setting to prioritize one band over the other. However, in our experience it is not possible to completely prevent the adapter from connecting to a band using the client settings alone, this is ultimately managed by the router. You may check with Debian* support if there are similar options from their side.
NOTE: Links to third party sites are being offered for your convenience and should not be viewed as an endorsement by Intel of the content, products, or services offered there.
Please let us know if there's anything else we can do for you.