ISP load balance application

Hi young19918 , 
Well, 
What are your configuration look like on Bigip. 

> Do you use irule to make the Source 192.168.40.0/24 go to Link A ?
> what are the pool configuration in this virtual server :
( Source: 192.168.40.0/24 destination: 0.0.0.0/0 pool member: A)


Do you use a pool has only " pool member A " if you do this , it's normal to see this behavior , and you should add pool member B to the pool that attached to "192.168.40.0/24 destination: 0.0.0.0/0" .

and you use another powerful way without irules to make the connections go to Pool member A , and when it fails connections will go to pool member B.
and when Pool member A returns UP , the Connection will return back to pool member A. 
This Powerful feature called : Priority Group Activation ( PGA ) on pool. 

So now you Plan of action (POA) : 
1) Create pool contains two members ( Pool member A , B ).
2) enable Priority Group activation on this pool , and use this Article : 
https://my.f5.com/manage/s/article/K13525153#:~:text=About%20Priority%20Group%20Activation%3A,a%20pool%20member's%20properties%20screen.

3) attach this pool to the Virtual server that hosts source ( 192.168.40.0/24). 
4) Test your implementation , by monitor that all traffic goes to Pool member A and then if Pool Member A disabled/Down , the new traffic will be directed to Pool member B , and if pool member A become up the new connections will go to pool member A again. 

Piece of Configuration : 
1) enable Priority group activation.
2) give Pool member A >>> 10 for example and give pool member B >>> 5 for example.
3) Select Less Than >>> 1 

Test it and let me know