If you have a community router in a shared building, you should check to find out whether you have been granted a single IP address (as you would normally be granted with a direct ISP connection) or a subnet.
If you have been granted a single IP address you must install a router in order to be able to connect more than one device to your internet connection. In this case you will have a double NAT situation and there is nothing that can be done within the confines of your own local network that can solve this.
However, if you have been allocated a subnet - say 192.168.1.0/24 (meaning that you can connect up to 253 devices) - then you do not need a router at all. Simply connect a switch (to allow multiple Ethernet devices to be connected to the single Ethernet port) and a WiFi access point.
Note: It is possible for a community router to allocate a subnet because each flat within the shared building can receive a different subnet - e.g. one flat may be allocated 192.168.0.0/24 and the next flat can be allocated 192.168.1.0/24 etc all the way up to 192.168.255.0/24.
Unfortunately, I suspect that you will only have been issued as single IP address as, from a network administration point of view, this is by far the easiest to setup and maintain as it requires nothing more than an ordinary consumer or ISP supplied router can accommodate. Allocating a different subnet for each flat/appartment requires more from a router and, generally speaking, it requires a higher level of knowledge from the administrator of the router - both during setup and for ongoing administration.
Edit (paragraph added): If you are lucky enough to be allocated a subnet rather than just a single ip address, and you use the switch/access point combination rather than a router to connect your devices, then your Roon Server may be able to configure a port forwarding rule using uPnP (if uPNP is enabled on the community router). However, if you can’t, your only option will be to get the community administrator to add a port forwarding rule for you (assuming you have no admin rights on this router). If that possibility is not offered, then you will not be able to get Port Forwarding to work and you will have to resort to using Tailscale (or similar). The community router (and it’s administrator) may restrict the port that you can use for Roon ARC. No two appartments can for connections on the same port - for example, if two appartments both use Roon, then it will not be possible for both appartments to use the default port 55000.
For the sake of clarity, it is not multiple DCHP servers that cause MultipleNatFound issues. It is, as the error implies, the presence of two or more layers of NAT (Network Address Translation).
NAT is the mechanism that allows you to connect multiple devices to a network when you have only been allocated a single IP address. In general, NAT works transparently for outgoing connections (Because the router doing the NAT has all the information required about how to route the outgoing messages and the incoming replies), but it does not work for incoming connections without the help of a port forwarding rule. The reason for this is simple - when the incoming connection is made, the connection specifies the WAN side ip address of the router, the protocol and the port number but the router does not know which of the devices on your local network (if any) should be used to handle the incoming connection - so the port forwarding rule tells it.
DHCP is, for IPv4, the mechanism by which devices on the local network are automatically allocated ip addresses within the subnet. It does not, itself, create the subnet - that is the purpose of NAT.
It is possible to set up a local network behind a router with no DHCP present at all. The Router would still perform NAT and, if there is another router upstrteam (including the gateway servers of an ISP performing CG-NAT [Carrier Grade Network Address Translation]), then you will still have a MultipleNatFound issue even though only one of the routers (the upstream one) is providing DHCP.
For IPv4, although not particularly sensible, this can be done by using static ip addresses for every device and disabling the DHCP service on the router (assuming your router allows it - some do - many don’t).
For IPv6, the equivalent DHCPv6 is not required (but can be used if desired). There are alternative methods of allocating ipv6 addresses using the facilities built into ICMPv6.