RT-AC68U AIMesh nodes keep going offline

I have two TM-AC1900’s that I flashed from RT-AC68U’s. I created a aimesh network. Works great. But when I let it alone for a while the nodes go offline. Any ideas as to how I can keep them online. I have all three modems using this version the the firmware.

Version : 3.0.0.4.384_20308-gead790e