Use the Image Builder to make an image without kmod-ath10k (ath10k firmware can also be removed).
It appears you have hardware failure in the 5 GHz radio, you're saying that 15.05 doesn't completely crash but still there's no 5 GHz.
Have you tried booting without the PCI card installed? That may cause ath10k to stop in a graceful way so that booting completes.
The C7V2 is a popular model-- one that is commonly recommended for its excellent OpenWrt compatibility. A lot of other people would have complained if there was something wrong with the releases that it wouldn't work on good hardware.
Of course know for sure what version you have (I think the conclusion has been reached that it is V2, this should be shown on the nameplate sticker) and use the matching build.