You need get the LEDE trunk and compile it.
git clone https://git.lede-project.org/source.git
scripts/feeds update -a
scripts/feeds install -a
sed -i 's/=4.4/=4.9/' target/linux/ramips/Makefile
echo 'CONFIG_TCP_CONG_BBR=y' >> target/linux/ramips/mt7621/config-4.9
git diff # confirm Makefile and config-4.9 changed by above two commands.
make menuconfig # select Target System (MediaTek Ralink MIPS), Subtarget (MT6721 based boards), Target Profile (MQMaker WiTi), and save, exit.
make -j1 V=s
You will get the firmware binary file in dir 'bin/targets/ramips/mt7621', upload file lede-ramips-mt7621-witi-squashfs-sysupgrade.bin to your WiTi's /tmp dir.
using 'sysupgrade -v /tmp/lede-ramips-mt7621-witi-squashfs-sysupgrade.bin' upgrade your WiTi and wait reboot.
and ssh into WiTi.
uname -a # you will see Linux Witi 4.9.20 #0 SMP Mon Apr 3 10:29:07 2017 mips GNU/Linux
echo 'net.ipv4.tcp_congestion_control=bbr' >> /etc/sysctl.d/local.conf
sysctl -w net.ipv4.tcp_congestion_control=bbr
sysctl -a | grep bbr # to ensure your ap turn on TCP BBR
Do not forget turn on TCP BBR on both side( WiTI and remote VPS server) to speed up your VPN connection.
If you want to switch back to kernel 4.4, recompile firmware:
git checkout target/linux/ramips/Makefile
make menuconfig # just Exit and select YES save.
make -j1 V=s
ssh into WiTi delete line 'net.ipv4.tcp_congestion_control=bbr' in /etc/sysctl.d/local.conf file, and flash the complied firmware back to WiTi, or any other firmware you want.