RMerlin,
I use rt-ac56u with the Russian ISP Beeline-Moscow, which uses l2tp with dual access.
Versions 3.0.0.4.374.32 and 3.0.0.4.372.31_2 don't work, log reads:
Unable to connect PPPoL2TP socket
Version 3.0.0.4.372.30_2 and current stock version 3.0.0.4.374.37 work perfectly.