As mentioned, leave the LTE setting on your phone enabled, just making sure that your phone isn't set exclusively to LTE (3g is required for voice service). Most phones these days don't allow you mistakenly it like that in the regular settings menu anyway.
Even if Public Mobile weren't allowing access to the LTE network (which they are), there still wouldn't be any need to disable it. The phone would automatically select the best possible connection available, regardless.
