When your Dodl charge is collected, your account cash balance will show a negative amount (however much of the charge is left to pay). The team will be in touch to ask you to add some cash to your account to cover this. They may also offer a friendly reminder to make sure you’ve always enough available cash to cover the charge each month.
If you can't add any more cash to your account, because you've used up your ISA or lifetime ISA allowance for the current tax year, you'll have to sell some of your investment(s) in order to make cash available to cover the amount due.
If you don’t keep on top of your charges, the team may need to sell some of your investments to cover what's due. That is a final resort though as no one wants this! See T&Cs for more details.