Annoying isn't it! However we have been doing extensive research and testing on this issue.
In summary it's an Android 7.0 bug. How do we know this? We actually found the 4 lines of Android source code that *fixed* this issue in a later patch version of Android 7.0.
In our testing on the Verizon network, we saw the issue 100% with all SMS / MMS apps (we tried the top 5 apps from Google Play), as well as Google's Android Messages and the pre-installed Messages and Verizon Message+ apps.
It's fairly easy to reproduce the issue, switch to the pre-installed Messages app, ask a friend to send you a rapid succession of group chats (MMS), and boom it 'locks up' and no MMS messages will be received (you'll just see the download icon in the bubbles, spinning for about 20 minutes). During this time you won't be able to send MMS either. Android seems to restart the culprit 'network process' after approx 15-20 minutes and then the messages will flow again.
The only 'real fix' is a firmware upgrade (controlled by your mobile carrier). Please understand this is not a Textra SMS specific issue.
However, given this Android 7.0 bug, there is little point Textra SMS raising "Cannot get MMS" / "Timeout" notifications as that's annoying. So in v3.38 of Textra SMS we will keep retrying for 30 minutes on Android 7.0 specifically, before raising that notification. This is *not* a fix, this is just to make the issue less noticeable. For example, if you are watching a Netflix show during a busy group chat period, then by time you take a look at your phone, most likely the messages have been retrieved without annoying you with multiple error notifications.
Also during this 'locked up' period, if you try to send an MMS, it'll still fail until the Android network process 'unlocks itself', typically around 15-20 minutes later. THERE IS NOTHING ANY APP can do about this, it's an Android 7.0 firmware bug. But at least you know what's happening now.
!! IMPORTANT !!: If you notice you cannot send MMS or are delayed in receiving them you can 'unstick' the Android 'locked up' network process by switching on airplane mode, then switching it off again. This resets the 'network connections'.