The john9527 fork worked great and now I have the repeater function. RT-AC68U_3.0.0.4_374.43_2-14E1j9527.trx is the one that I used to get it to work. THANK YOU for your advice and john9527 for the flash!
braebyrn and all: were you able to get the repeater to function properly? I followed the very helpful guides and was able to downgrade TM firmware, then flash Asus CFE, then flash Asus firmware, then flashed Merlin. The repeater option became available and I was able to select my existing AP. After the ac1900 configured it self I was able to see and connect to it but not use the internet. In the configuration I am able to see that it is indeed connected to my existing AP. I thought it may be Merlin so I flashed to John's fork 15e1, but received the same results.
What am I doing wrong?
Thanks a bunch to the contributors btw. I had zero knowledge this AM and was able to flash using your guides.