Is this the first time they have been late? At least they texted you, which is one point in their favour. I would have a chat and point out that you do need the money to pay the mortgage so you definately can't allow anything past the 21st.
Not sure if you could give them a letter saying you will allow them until 21st, but failure to pay on that days will result in notice being effective immediately. Maybe PRTB can help with this, but not sure if they will gove you an answer over the phone.