Hmmm - How to explain this without going too deep down the rabbit hole....
WiFi is not ethernet... it has concepts of station and AP, where in etherenet, everyone's equal. In standard 802.11 (wifi), the AP only knows about / works with the MAC addresses of stations that are associated with it - so if a station tries to act as a simple bridge, then it won't work because the AP isn't going to honor the MAC addresses of devices behind the bridge (those MACs aren't associated stations).
The GoPro's AP is probably very very basic, so it's not going to offer bridge mode or WDS options.
Therefore, the Mikrotik repeater must be able to fake it all. Making matters worse is that you have the same things to deal with on the main WiFi which leads to your repeater...
You're going to need true bridging between the repeater and the real AP, and pseudobridge between the repeater and the GoPro, so that the NetMetal can "masquerade" the MAC addresses of lan devices when it repeats to the GoPro.
EDIT: I just realized that the Netmetal IS the main AP for the Android phone... so basically, I think setting the wlan2 to mode station pseudobridge or pseudobridge-clone might do the trick. You'll need to add both wlan interfaces to the same bridge, of course.
When given a spoon,
you should not cling to your fork.
The soup will get cold.