The isl file is the license file, and it can only be provided to you by the vendor who "owns" the AMC license record.
If you want to get the license file, you would have to either deal with that vendor or move your license record to another vendor who is willing to provide it for you. The new vendor would send you some docusign documents to fill out online so they can request an ownership change with Mitel, and the losing vendor will have the opportunity to respond or dispute the move (often occurs in situations where you are not in good standing with the losing vendor, like you owe them money). If that is successful and approved by Mitel, the winning vendor can download the license key and provide it to you.
Unfortunately, there is no other way around this... The system requires a valid license that is associated with the license dongle (maybe physical device or embedded in the compact flash, depending on the age of the device). Otherwise it will reboot every 240 minutes until a proper license is loaded.
