iPhone Stolen: Still Offline in Find My After a Month - What Should I Do?

There is nothing more that you can do, other than

• Filing a police report
• Asking your carrier to blacklist the phone – which may interfere with its ability to communicate with the Internet, but, like Activation Lock, is a way of helping to turn the phone into a useless "brick".
• Hoping that the phone was locked when stolen, and that the thieves do not know your passcode. (Probably the case, if it is still associated with your Apple ID.)
• Not falling for "phishing" attempts which may try to get you to provide your credentials (so that the thieves can get into your phone) or to remove the phone from the list of devices associated with your Apple ID (clearing the Activation Lock and letting the thieves "make your phone their own")

Chances are you will never get the phone back - and may never even see the location again. At this point it's about using passcodes, Lost mode, remote Erase requests, and carrier "blacklisting" to keep the criminals from accessing your data, and to "brick" the phone so they can't reset it to make it a usable phone again.