What most people do is buy the Intel Wireless-AC 9260 802.11ac (or some other miniPCIe WiFi card).
While you still have the old Atheros ath9k card in your Librem 15, install the proprietary firmware for the Intel WiFi card:
wget http://ftp.us.debian.org/debian/pool/non-free/f/firmware-nonfree/firmware-iwlwifi_20190114-2_all.deb
sudo dpkg -i firmware-iwlwifi_20190114-2_all.deb
That should solve your problem.