I don’t use more than 1 eth port on my DS216+, running 2 nodes on 2 HDDs, but I had QUICK missconfigured when I tryed using the same 28967 internal port for both. So my advise is to use different ports, internal and external, in router and in docker run, for each node, for tcp, udp, dashboard. Using this config I never had a problem. I tryed all the combinations, getting quick missconfigured with others. Docker and DSM ar up-to-date, docker ethernet runs in bridge mode (default).
https://forum.storj.io/t/my-docker-run-commands-for-multinodes-on-synology-nas/22034
I must add to this the tcp_fastopen line in the kernel setup. Also I set the kernel parameters, including tcp fastopen, as described above, to run at boot. When I have time, I will update that post with screenshots, step-by-step.