MD5 crack with first chars known

Hi,
 i'm trying to decripty an md5 hash, i at last found the right hash ad how it's done.

I know that my hash is created with this formula MD5(user:modem model:password), i have created a test user called test with password test and MD5(test:modem model:test) made exactly the hash stored in the db.

Now i'm trying to use hashcat to recover another user password, so i have a fixed part that is "another user:modem model:" + the brute force part.

I' reading the mask attack and so on but i must do something wrong, tips will be appreciated.

Thanks in advance.


(modem model could have blank spaces, just for info)