As a general thing, you are allowed to tell people
the "bare" facts that you are leaving, and where your new job is.
If they choose, as customers, to follow you, that's their choice.
Note that they are not "your" customers,
they are your (ex-) employer's customers.
Even the ones who followed you from any previous workplace.
But if there is any more than a bare statement of fact,
then you may be in breach of your duty to your (ex-) employer
not to do anything to harm them.
That you signed or didn't sign anything is irrelevant.
Your (ex-)employer will probably be cranky
(think how you would feel if you were the employer!).
And they might make threats and talk about suing you
and all that sort sort of thing.
If they do, just let 'em vent. It's a matter for them
to demonstrate any loss.
If they defame you, or your new employer,
and you suffer damage to your reputation personally,
or your employer suffers commercial loss as a result,
then you may have further options.