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

Could be multiple reasons, but usually indicates an inability of the two devices to communicate (access control or routing).

First, are you adding the LTM pair as a BIG-IP System (Redundant)? It should be done this way rather than as individual BIG-IP systems.

In order to see what is happening, try running iqdump to see if your GTM can talk to your LTM. Run this from the GTM CLI (bash prompt):

iqdump LTMipaddress