From the RRG:
A unit that has a weapon with the cumbersome keyword
cannot both move and attack using that weapon during the
same activation, unless the move is a pivot.
• If a unit performs an attack with a weapon that has the
cumbersome keyword during its activation, it cannot
perform any moves during that activation, except for pivots.
• If a unit performs a move, except for a pivot, during its
activation, it cannot perform an attack with a weapon that
has the cumbersome keyword during that activation.
• A unit can use a weapon with the cumbersome keyword
during an attack triggered by spending a standby token, even if
that unit moved during its activation earlier in the same round.