Call the support, tell them this problem, they have a beta for it.

Your thread title says mr3420 but on your post you mention the mr3220 model, which one is it? I have a mr3420 and i've had this problem for months now, even before i updated to the latest firmware, it seems that dlna stops working when tv is connected wirelessly, i have another smart tv connected via lan cable where dlna works properly.
Someone suggested on another forum to just flash the router with thirdparty firmare, I was hoping to avoid doing that since theres a chance of bricking the router itself.