Hi,
This has been discussed in this thread: Archer C7 2.4 GHz wireless dies in 24~48 hours
Basically, ath9k firmware blobs are buggy and will make 2.4GHz go down after a while. Usually, this will happen when there are many clients connected and lot's of data being transferred. There was a hearsay that backdating ath10 (5GHz) drivers might help with 2.4GHz stability but that has been debunked since. I tried it personally.
People have made scripts that tail the log and reboot 2.4G when it gets stuck but currently there is no definite answer on how to fix it permanently.