You may attempt to clear device cache via recovery mode. Though you state you are not interested in a factory reset, it is likely the next logical troubleshooting step if you have both reset network settings and cleared device cache.
Though you are in the US, M series devices are not broadly available in the US and are targeted toward emergent markets, particularly in southeast Asia. Users in those regions who are more familiar with M series devices may be of more assistance to you.
You may not be interested in a factory reset (no one ever is), but sometimes it's the only fix. If you are unwilling to take suggestions, which may include the suggestion to reset, no one can help you.
