SMS and MMS work a bit differently on R.W. than standard carriers.
I believe that all SMS and MMS messages are routed through and stored on R.W. servers, thus enabling the Anywhere app to sync the last 30 days of messages sent to and from the phone.
You can log in and out of the Anywhere app on PC and phone just fine, enabling you to switch to a different R.W account to send and receive message on that number.
But any texting app you use will have to communicate with the same R.W. sever anyway.
Signal is an app that has its own special messaging thing for security, but it, and other apps like it, require both parties to use the app in the same configuration. I do not know if this kinda thing will even work on R.W., but if it does, it would probably count towards your data allotment, and not as part of the “unlimited talk/text”.
There is other apps that use its own platform for sending messages, Hangouts, FaceBook, Skype…etc.