For you next migration project: You could try to temporarily turn off CLR strict security on SQL Server 2017 and then re-enable it after your FNMS Database migration scripts have completed?
EXEC sp_configure 'show advanced options', 1
EXEC sp_configure 'clr strict security', 0;
Query from @erwinlindemann didn't help me, but running this query helped me:
ALTER DATABASE [FNMSCompliance] SET TRUSTWORTHY ON
EXEC sp_changedbowner 'sa'