rs232-ttl converter problem
So it was a bad design in the end, the unused Tx can't be left floating, yet all of the modules I tried had the unused Tx floating (MAX232 has 2 channels, but the modules use just 1 channel) .
If they are floating, they will act as antennas if there are RF signals, and will latch the IC, thus heating in the process.
Used the TI galvanic isolated TTL-RS232 transceiver instead, made a PCB with it and now the communication works flawlessly.
http://www.ti.com/lit/ug/tidu298/tidu298.pdf
In this place it's explained why it is bad design, and it's about the same problem I had, just in a slightly different situation
http://www.edaboard.com/threads/352767.html
Extra advice: if you are planning to transmit data through RF and you will be placing the modem relatively close to your transmitting TTL device, by all means try to get a modem that has an interface at TTL levels, adding a RS232 interface in the middle also adds a possible point of failure, plus it costs more.