@bobstc, if you've verified you have flashed the correct 386.xx version and you have downloaded the correct version of 388.xx for your router (and the router still doesn't upgrade to the new firmware), then this is what I would do next.
- Test with another browser, or a browser in 'safe', 'incognito', or 'InPrivate' mode.
- Remove all cables and USB devices from the router.
- Reboot the router via the GUI (yes, via the WiFi, with no WAN or LAN cables connected). Does the upgrade work now?
- If not, pull the power from the router and the wall ac outlet. Leave the router 'on' but unplugged for at least a minute or two. After you have plugged in the power supply to the wall and the router, and the router has fully booted up, does the upgrade process work now?
- If not, perform a full reset via the GUI and make sure you check the 'Initialize all settings...' check box before you hit reset.
- After the router has rebooted, do a temporary setup (just so the router doesn't show the setup screen after a reboot again and you won't know if you did a reset or not, using this different method). Then, use the Reset Button to reset it again. If after the reset the setup screen doesn't come up, you did not do a reset properly using the Reset Button.
- Finally, do one more temporary setup, then use the WPS Button method (see link below), and be sure the router was really reset (by verifying it comes up with the setup screen when you try to log in after it has booted up).
[Wireless] ASUS router Hard Factory Reset | Official Support | ASUS Global
If you followed this sequence precisely, and the router still isn't accepting a properly downloaded, verified, and matched firmware for your model router, I would consider this a hardware issue of the affected router.
Even doing all the above, before I would attempt to return it (if the return window is on your side), I would wait for Beta 3 from RMerlin before giving up on the hardware.
Hope this helps a little.