How to print at low temperatures (filament melting at about 70 °C)?
70 °C is a specialty filament. It is well below the MIN_TEMP defined in any sane firmware. In Marlin, you can't turn on the extruder in any way, while this is online.
You do need to define your firmware to allow such a print - either by dropping the value in the firmware or disabling Mintemp-protection and then flashing that firmware. That is quite invasive.
To temporarily disable the MIN_TEMP, you need to run the G-code M302 P1 or M302 S0 - M302 on its own does nothing. However, some firmware distributions might explicitly prevent these two commands of turning off the check.
In that case, you might use M302 S65 or similar to drop the MIN_TEMP.