Hi - unfortunately that doesn’t change anything 
I can edit the interfaces as you describe, but it still won’t connect to the AP on the DFS port.
Also the setting is the same as editing /etc/config/wireless it just puts the country code in (“GB” for me).
Looks like this is a “Repeater” mode limitation, it simply won’t work with DFS channels. GL-A1300 - How to disable DFS validation? - #4 by AnotherPhil seems to confirm my experience.