I upgraded from 18.06.5 to 19.07.0 on my TP-LINK Archer C7 v4 . After it rebooted radio 0 (5 GHZ) worked but radio 1 (2.4 GHZ) would not start . I 'restarted' it / rebooted / cycled power but no joy .
Finally I went back to 18.06.5 (the best way to do this is another question) and both radios are working .
Did you choose to save the settings by chance? If you did then I suggest that you flash again without using the settings, or from the terminal using the following command.
This is expected when changing from ar71xx to ath79. The wifi configuration needs to be re-generated. If you don't mind losing other settings (which is a good idea, since some others could be broken too), either reset to defaults on the GUI firmware page or run firstboot on the CLI. It isn't necessary to do a complete re-flash.
Look if a "radio2" has been added, that's actually your 2.4 radio which has replaced the misconfigured radio1.