The easy way is also the most expensive. I tend to use this approach for large corporates.
Use Cisco Duo for your MFA (you really should - it is so good!). You need the "Beyond" plan.
https://duo.com/editions-and-pricing
Deploy the Duo Mobile client to all your devices as part of the profile. You'll need this anyway for MFA.
Then create a Duo device trust policy, saying that Office 365 can only be accessed from trusted devices, and mark all devices with the Duo Health agent (which is included in Duo Mobile) as trusted. For bonus points, you can also specify things like minimum OS version, browser versions, etc.
You can also use the same approach to limit access from corporate-owned Windows and Mac computers. On computers, you can also specify cool things like saying the computer must have the corporate antimalware solution installed before being able to access corporate resources.
https://duo.com/docs/trusted-endpoints