I think you are right that you can’t get a single IP address for your app (I would check with support to confirm because this is v3, and could be a different case). Is it possible to change the authentication method of the web service that you are using? Maybe use a client certificate or JWT? Another option you can look into sine you are on v3 is a vpn connection, the documentation says its deprecated but I remember that was possible in v3.
You can find the Mendix IP addresses here: https://docs.mendix.com/developerportal/deploy/mendix-ip-addresses#2-definitions
They are subject to potential change, but Mendix should give you 48 hours notice if this is going to happen.
As you said it’s a shared range, but I think it’s the best you’ll get at the moment.