* ok, my principal "thread" for audio was for 2 cameras IPM-721W on other home with other router
* But i make your test with line or micro in my home with my Asus RT-AC86U router with 2 camera, one amcrest IP2M-841W and one old Foscam (your line audio choice working) thes test is for my amcrest only and its normal. And for my home camera (amcrest and foscam) i using tinyCam pro, with no DNS for my cell and samsung galaxy pad.
For my case is more hard make setup with reserved IP and port only, Foscam setup 2 port only and amcrest 4 and more inside setup, i am not bad but not wan and lan spécialist