The APN bettings are in the mobile network settings, no dialer code should be needed. If you are are using CDMA, the ROM tends to default to Sprint settings, and the settings for Ting need to have 'default' added to one of the settings.
The trick is getting the APN menu to show the options as it tends to lock them out once it connects, even poorly, to a carrier network. Sometimes it takes a few reboots, I had to use a dialer code to disable the LTE radio last time to get the APN menu to show any options. Once I could edit the Ting settings and turned the LTE radio back on, it worked.