Maintenance when nrp has gone back to university

[Whyisthissomuchmoredifficultthanitshouldbe]

Question if anyone could help please! Nrp has gone back to uni, initially told me it is a part time course and still working. Now stopped paying maintenance and claiming it's a full time course. Still earning but less than before, but theres a long history of lies and feigning ignorance to get out of things. I'm waiting on a cms calculation, but if it's true that it's a full time course does that mean no maintenance is due? Nrp has never paid what the cms calculator has said. I never submitted cms application before as it wasn't worth the hassle (coercive relationship and would have had a negative impact on the kids, and I could get by without it).
If anyone has been in this situation it would be great to hear how cms handled it - otherwise I'll just wait for their calculation. I assume nrp would have to prove it was a full time course? I've tried to Google it but all the answers are about the kids going to uni not the parent 🤦‍♀️