RT-AC1900P crazy D/L slowdown with modem in Bridge Mode

Hi, First time post here. I have CenturyLink internet only DSL, advertised as 20Mbps D/L, 0.7Mbps U/L.
I have a Zyxel C2100Z (own it, not rented/purchased from CL). DSLreports shows D/L 20+, U/L 1+ so no
issues. I have a VOIP phone (Yealink T21P-E2) that works flawlessly with the Zyxel C2100Z, but my
VOIP provider (Flowroute) doesn't offer voicemail. So I got a great deal on an Asus RT-AC1900P that
appeared to be brand new. I flashed Merlin (currently have latest 384.13 installed) and...

RT-AC1900P crazy D/L slowdown with modem in Bridge Mode