i have 2 mikrotik router rb433 working with same configuration both with 12v poe, one with 10 mts cable and the second with 30mts cable, the 10 mts cable one has been up for over 6 month working just fine, the 30 mts cable one we just installed and it reboots itself every 60 sec. any suggestion? should i lower the cable length, or perhaps upgrade to other firmware

I would try a 24 volt power supply. 12 volts is too close to the bottom limit. There is voltage drop on long cables so you might be under the required startup voltage by the time you run 30 meters.

Tom