Synology to Edgerouter X vPN problem

I am trying to setup a VPN between my Synology and my Edgerouter X but I cannot make it work. Here is what I have done so far.

I have setup a L2TP VPN server on my Edgerouter. I can connect to this VPN from my phone with no problem and get access to all the resources on my internal network.

I configured the same VPN on my remote synology diskstation. It connects, but I cannot reach it from my network. Web GUI does not respond and I cannot ping the IP address.

If I open a terminal on the Edgerouter I can see both my phone and the Synology are connected. I can ping both the phone and the Synology from here. However, from my PC I can only ping my phone and not the Synology.

What are my next steps? How do I troubleshoot from here?