$1/mo is prevented by a hard minimum, and likely for good reason as the credit card processor imposes a flat base fee.

I don't really want to see a bill for e.g. £0.41 every month on my credit card statement, and at some point the transaction fees would become a bother for Signal.

I'd have ticked "recurring annually" if the option had existed.

Except you have to be careful so the monthly amounts don't end up so small that the credit card processing fees eat up the majority of your donation! The processing done via Stripe (which is used by Donorbox) has a fixed minimum fee of 30 cents per transaction.

Edit: unless they have some special lower cost stripe rate of course, in which case you can ignore my comment altogether ;)