Thanks @pert, appreciate the tip.
But to get back to the root of the problem, how do I TX the serial data to a sim800C modem in the correct way? I'm dealing with IOT and TX / RX from Ubidots can get tricky if the strings get larger that 256 bytes, which is the case most of the time.
Like I mentioned earlier in this thread I tried to slow down the transmission to the modem with different time settings, but nothing.