Load Balance between two ISPs Using LTM .

If you have the DNS module then the link monitoring is the way:

https://support.f5.com/csp/article/K6848

If not then maybe you can test with a transperant monitor and check a specific destination by going through the two ISP:

https://support.f5.com/csp/article/K8971

Also if one ISP should be active and the other standby check the F5 priority group option:

https://support.f5.com/csp/article/K13525153