We have covered this in other threads. The B1300/S1300 routers are dependent on the Qualcomm build environment which is currently stuck on 15.05.1, nothing that GL can do at the moment about upgrading to a later OpenWRT version.
As soon as Qualcomm updates their build environment, GL will put up an update shortly after.
If you want you can build your own 18.06.2 following the OpenWRT build guides, but you will loose a lot of wifi performance since it won’t contain the Qualcomm closed source drivers.