The second AP is connect via homeplug TP Link WPA4220 kit which has wireless capabilities. My phone & laptops connect to that AP with full speed.

I have tried turning off the wireless on 2830 to see if any difference.. none
I have changed channels on both the TP link & draytek with various combinations to no avail
Wan1 & Wan3 are disabled as the 2830 is connect via WAN2 form openreach modem.

I am running 3.6.7_sb_232201 firmware.

I am at a lost now. I can just disable the wireless feature on the 2830 and purchase another TP Link 4220 to keep the speed up and plug it in next to the draytek.

How far from the 2830 are you getting the speed?
Also have you ever loaded by mistake the firmware from single or dual band models? (wrong firmware), this may cause issues to the wifi driver

when i was beta tester with virgin media testing their new SH2 they do let us use and our own routers
I had speed of 80-90 mbps via wifi out of 100 at 1 and half meter away. that time was with firmware 3.6.3 & 3.6.4, i am not any longer with VM to confirm newer firmwares speed but now with adsl i am getting full speed (that is not much anyway)
If you going to try older firmwares 3.6.3 i remember had issues with android devices when was connected to the wifi (also the rst file will need to use)

I am only less than 1m away form the 2830. I have also tried 10m away.

Never loaded dual band by mistake, only single band but have never loaded a .rst file.

Can I backup my settings than flash firmware with an .rst file then restore my backup configuration?

is not recommend flashing the firmware and use so long gap with firmwares for backup files because it could make conflicts due to new / old features gap of the firmwares ... but if i was having the issue i would try just to test the wireless speed only and if was fine i would master reset again the downgrade firmware was worked and do from the begin all settings, if i have explained in clear way

Also after load the downgrade firmware you just can set only the ips parameters (2 min work just the internet to come back on) and then just use the wifi to test speeds, remember to disable one wan for not stolen cpu memory
First Keep a backup of the 3.6.7 firmware if you came back to it just to load easy your settings

Ok, thank you will try to downgrade the firmware, but cannot find old firmware files?

I think is worth to try
Please note this link is for the Single Band Units ONLY
ftp://ftp.draytek.com/Vigor2830/Firmware/Single%20band/