I have used the 5V pin instead of the Vcc pin to operate some sensors on the Arduino MKR GSM 1400. Altough the operating voltage is 3.3V.Because of that it doesn't work properly anymore, some analog sensor values are clearly wrong and the GSM module works only occasionally. Is there any way to repair this myself or send is somewhere, or should I buy a new one?
Feeding 5 volts into an input most likely cause the protection diode on that input to conduct the 5 volts onto the 3.3 volt supply. That in turn would exceed the voltage on all the 3.3 volt connected parts. Likely to cause more damage than just to the affected input.
The cost of shipping and labour to repair the board would exceed the cost of buying a new replacement board. A repair would require too many parts to be replace. And replacement would be required of even parts that might appear to work because the risk of failure would be high.
Buy a new board and be confident that it will work.