GTM adding LTM as server - Reason no reply from big3d : timed out

Yes I am adding the LTM pair as a redundant pair. Actually if I remove all the manually created VS under the GTM group then the server is shown as green (this is probably default behavior if there is no VS to check I presume). So it does look like it's working but it is odd that no VS is being detected under that LTM group (unless it only detects a very specific type/class of VS).

I also tried the IQDUMP as you suggested but it is not that clear what I should be looking for, I do see that under "server", there are traffic and it shows a status of 1 as active.