Question in title.
I have a drone I need to test which requires a 5GHz connection.
If it's possible to add 5GHz wifi capability through a dongle then I want to give it a go.
No. It's not possible
Your device doesn't support dual band (802.11 a or ac not supported) and adding OTG dongle that supports dual band isn't going to help. As an analogy, your service provider may provide WiFi but if your router ( like the older ones) doesn't support 5Ghz you can't use it in upper band