Two or more routers in network causes problems for Synology NAS
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Hi there!
Let me begin by saying that I'm no networking pro, I just learn by trial and error. 
I wanted to set up a mesh WiFi network for the first time and went with the Deco M9 Plus. Setup went fine and everything seemed to work perfectly. That's until I tried to link my Synology NAS to this network. I get the warning that there are two or more routers in my network, leaving me unable to set it up so that I can access it from outside my network.
My ISP modem is probably the "second" router it's referring to. I've disabled WiFi on that device, but can't set it to bridge mode, so I'm not sure what device is handling NAT. UPNP is only enabled on the main Deco, not on the modem. My setup is as follows:
ISP Modem (192.168.0.1) ==> DecoM9+ (192.168.1.1) ==> TL-SG108 Switch1 ==> DecoM9+ ==> TL-SG108 Switch2 ==> Synology NAS (192.168.1.105)
The first Deco is the main one downstairs, all of the devices downstairs are connected to Switch1. There's a cable going upstairs from Switch1 to the second Deco. The devices upstairs, including my NAS, are connected to that Deco through Switch2. There's a third Deco, but that one's connected over WiFi.
Are there any ideas how this could be fixed?
Thanks in advance!
@terziyski That was my backup option for when there is no other possible solution. 
Because in that case, I won't have the perks of de Deco functionalities (like Parental controls, QoS, connection alerts, ...). So I'm looking for a solution where I can keep those perks.
