GTM ISP 1 and ISP2 Monitor

HI Expert , 

I have Implement GTM with LTM 

LTM has virtual server with Private IP.

how can we let the GTM reponse the subdomain wide ip with Public ip (ISP1 or ISP2).

if ISP1 down , GTM will detect and will resolve with ISP2 public ip, as i test , i only get private ip address not public ip.

base on checking, seem we need to write irule , can you provide me the script and if have video , that would be a great help.

Appreciate.