Thanks for the help. Made a lot of progress and learnt about the manifest and WiFi Debugging!
Turns out the problem was my own impatience – I just had to wait a little longer for the game to load. I had assumed it was the ‘no signature’ issue I was experiencing earlier so quit too early thinking it wasn’t working.
Anyway, now I’m experiencing this issue instead:
Guess it could be an incompatibility with Exynos - but other games are working fine, so I assume this can be fixed somehow.