As @dirkt stated in the comments, my requirement is not an easy one. But I managed to find a solution.

At first, I replaced the third router standing between two networks with pfsense installed on an old pc.

pfsense comes with IGMP proxy for managing multicast routing. Enabling it didn't work for me.

So I searched for other multicast routing solutions and I found pimpd.

I installed it manually to pfsense as it is not included in official packages, and started the server using default configuration file and voila, DLNA servers are discovered, with a delay I have to admit. This may be improved by messing with the configuration.

Note: I would leave the question unanswered hoping for better answers.

As @dirkt stated in the comments, my requirement is not an easy one. But I managed to find a solution.

At first, I replaced the third router standing between two networks with pfsense installed on an old pc.

pfsense comes with IGMP proxy for managing multicast routing. Enabling it didn't work for me.

So I searched for other multicast routing solutions and I found pimpd.

I installed it manually to pfsense as it is not included in official packages, and started the server using default configuration file and voila, DLNA servers are discovered, with a delay I have to admit. This may be improved by messing with the configuration.

Note: I would not accept my answer, hoping for a better one.