You cannot directly call .NET DLLs from al codeunit or extensions, because Saas service doesn't allow you to use local resources.
So you have to call Dlls from azure function. In the link above, it shows how to integrate a DLL in azure functions and call it with Al language.
-----------------------------------------
Best Regards,
Salah Eddine BENOUALA
Dynamics NAV & 365 Business Central Consultant
Please verify my answer, if you find it helpful.
Doing so you'll show others that there was found a solution and you credit my help.