@UK Sentinel Have you some news for me?
I emailed ASUS again yesterday but have not had a response, 
Ok, so I have had feedback from ASUS and unfortunately there is no new firmware or beta’s available, I stressed to ASUS that the last firmware is nearly 2 years old and this has now been raised internally within the ASUS team that work on the 4G-AC53U.
I also asked when the next firmware release was due and was advised model is still supported but no plans/schedule for new firmware currently exist.
If your 4G-AC53U has a feedback form, I can only advise all 4G-AC53U owners to use this to raise this issue with ASUS directly,
The more owners that ask for new firmware, the more likely they are to produce and release it 
In a completely sane world, madness is the only freedom (J.G.Ballard).