Text START (in all caps) to 57439 from the same phone you're using for your Split Pay account. This re-subscribes you and login codes will start coming through again.
Once you've sent START, try logging in again. The code should arrive within 30 seconds to a few minutes. If it doesn't, see the FAQ on I'm not getting the text with my Split Pay login code. What do I do?.
A heads-up: texting START restores all our transactional, account related texts. lt will not opt you in to receive marketing SMS's if you've unsubscribed there.