I know it’s been a very long time since you asked this question, but in case someone else stumbles across it, here is an important caveat—to synch Grids to external clock, you MUST remove all CV inputs, press the button until it lights up steadily, make the relevant changes using the knobs, then press the button until it turns off. Once that it done, reattach clock input and any other CV inputs. Finally, spin the tempo knob all the way counter-clockwise and leave it there.
At this point, your Grids will be synched to external clock source. Unfortunately, you will lose the synch, if you turn off the Grids, then turn it back on.
If you use multiple Grids synched to the external clock, as I do, note that if you divide the clock going into one of the units, using something like an Ultima Ratio module, but then want to realign the synch to match other Grids, it may take a bit of twiddling to nail the realignment. Play with this and you’ll find what works.