I've got an unrooted Samsung Galaxy S5 from T-Mobile running Marshmallow (6.0.1), and am trying to add an APN so as to use the phone with FreedomPop. I've installed the FP SIM card and entered the unlock code. The next step is to add an APN for FP, but when I try to access the relevant settings (Settings→Connections→More networks→Mobile networks), "Mobile networks" is disabled (grey text, tapping has no effect). How can I enable the settings (the preferable route), or, failing that, add an APN in another way? (So far, the other methods I've seen require other steps (e.g. rooting the phone) which have their own roadblocks.)
"Mobile networks" remains disabled after switching back to the T-Mobile SIM card. The phone lists the ICCID for each card when inserted.