Android devices and iOS devices do not connect using the same connection protocol, so in some situations one device will work while the other doesn’t. Android uses openVPN while iOS uses IPSEC. Some connections may block one of these connection types but not the other. To test if this is the case, connect to a different WiFi connection or with your mobile data to see if the app will work.
If you have admin rights to your router, you may be able to change the settings to allow for both connection protocols to be allowed. You may, however, need some assistance from the manufacturer of your router.