When you initially agreed to use SoberLink you didn't know he was going to screw up 1/10 of the time. You expected 100% compliance. Now that you know he is not willing/able to be 100% compliant, some modification of the parenting agreement is necessary.
Can you propose a "laddered" approach to parenting and SoberLink? Start with supervised visits only. If he manages x weeks with no "fails" (missed tests or positive tests), he "graduates" to the next stage, short unsupervised visits. If he "fails" in this stage, he goes back down to supervised and has to complete x+y weeks of compliance before he can get back up to unsupervised. It's like Snakes and Ladders. This provides him with rewards or incentives for using SoberLink correctly and makes it clear that your objective is not to cut him off completely from the kids (even if that's what you would secretly like to happen). That's the arrangement I've got, which was just signed into a court order by a judge.