I hate to zombie this thread, but I've come up with a solution and I figured I'd share.
I've got Marshmallow, and so no visibility to a setting for Connection Optimizer. I thought that was long gone, but it turns out it lived on, in the ItsOn service.
ItsOn is part of Sprint's software package (a technology pulled from ItsOn corporation, which makes software to help manage wireless networks) and originally was billed as part of their drive to help personalize their PrePaid services. It lives in the application: NetworkSystemProvider which may show up twice in one's application list. You can see its availability by using the ##3282# dialer command (##DATA#), and going to "NetworkSystemProvider" and tapping on it, to reveal "ItsOn" and an On/Off radio button. If you have your MSL you can choose "off" in there. However, I didn't want to get my MSL, or call Sprint for it, so I found an easier way.
This thread: http://forum.xda-developers.com/spri...t3100303/page2
- talked about deleting a bunch of files in /carrier/ and /system/ related to ItsOn. But I found an easier and less heavy solution.
In /carrier/ on the root of the phone, should be located a file titled "itson_enabled" -- on a whim, I renamed it to "itson_disabled", using a Root File Explorer. I then rebooted my phone.
After doing so, ItsOn in the ##DATA# screen showed "Off".
And to my delight, WiFi Tether Router remained stable!
This solution is easily reversible by renaming _disabled back to _enabled.
Hope this helps!