ASUS RT-N15U (the ASUSWRT Firmware)
Courtesy & thanks to ASUS, finally the WRT is available for this Broadcom device, cool ! 
Of course, 3 digits port-based multiple vlans are supported 
Finally the QoS is working !!
WiFi Testing
Test copy a huge file from NAS
WiFi speed test result, Write/Read a 2GB file size over my LAN's NAS
Due to it's hidden antenna, overall the wireless performance is acceptable.
Tested on same laptop, I feel like the recent test on the RT-N12-C1 ASUSWRT is slightly better 
Basic Setup for UniFi
Setup may refer to the recent test on RT-N12-C1 ASUSWRT setup section
Overall I tested it for more than 3 days with various type devices connected, no problem encountered, this beta firmware is good to go !!
Before you proceed further, please read and make sure you fully understand that:
- I tested this RT-N15U beta ASUSWRT firmware on my Huawei HG850a BTU with VIP5 package, it should work on other BTU/VDSL2 as well
- This firmware is made available on an As-Is basis, I shall not be liable for any damages cause to your device either before, during or after flashing the firmware.
- I don't create & own the firmware, the firmware was released by ASUS unofficially, so please understand I will not entertain any email enquiry or support
Good luck & enjoy !! 
RT-N15U ASUSWRT FW 9.0.0.4.309 download HERE