I'm having the same problem (I think). I cannot send or receive MMS messages through Hangouts while my phone is connected to Wi-Fi.
I'm on T-Mobile, and I thought Wi-Fi calling might be the issue. Doesn't seem to be the case though.
Here's what I've tried for Hangouts:
-Wi-Fi on and connected - Wi-Fi calling enabled and set to Wi-Fi Preferred [FAIL]
-Wi-Fi on and connected - Wi-Fi calling enabled and set to Cellular Network Preferred [FAIL]
-Wi-Fi on and connected - Wi-Fi calling off [FAIL]
-Wi-Fi off [SUCCESS]
I had the M8 on T-Mobile and had to return the phone because of this limitation. It exists an all M8's regardless of carrier.
I decided to contact HTC support and the rep confirmed that you MUST be connected to a mobile network to use group messaging. This is truly an oversight by HTC. I don't have this issue on any other phone.
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