Why won't an SMS go through in certain geographical areas on my iPhone 5s if it contains exactly 148 or 149 characters?
It looks to me like your texts are approaching a length where the phone needs to send a concatenated SMS. Judging from your success with 3G and not on LTE I would guess at this being an operator problem with supporting concatenation on LTE in some scenarios.
EDIT: The test you ran with your friend connected to the same tower obviously puts this to some doubt.