I don't know the answer.
If only the personal iOS devices have a certificate deployed, you might be able to enable certificate-based authentication in O365.
Typically I would solve something like this using Cisco Duo and the trusted endpoints feature.
https://duo.com/docs/trusted-endpoints
A less strong method would be to buy Cisco Umbrella SIG, run all of those machines traffic cthrough Umbrella, and create a conditional access policy to match the ranges that the IP traffic is coming from.
https://support.umbrella.com/hc/en-us/articles/360059292052-Additional-Egress-IP-Address-Range
Another option would be to make the devices use a full tunnel back to a Meraki MX - so all of theitr traffic appears to come from one IP - and match on that IP address in conditional access.