Add a ContentType to list when threshold is exceeded

Hello,

on SharePointOnline I try to add a second SiteContenttype to a list with already 30.000 item. 

But using the browser either  using pnp i am successful. I always get the "The attempted operation is prohibited because it exceeds the list view threshold." error. How to add a ContentType to this list without removing the items?
Thanks,

Lutz