Couchbase XDCR getsockopt: no route to host error

I’m trying to make an XDCR replication between two Couchbase clusters. I’m using the following setup:

Ubuntu 14.04.5 Server on each node. 4 x Nodes in each cluster for a total of 8 nodes. I have an different ISP for each cluster. I have an Synology Router in front of each cluster. I’m using Couchbase 4.5.1-2844 Community Edition.

I have setup an cluster reference but when I try to create a replication I get the following error:

Attention - 2017-12-06 12:50:21 909fe0f4039f47383f11bd96b32b87e9/default/default failed to start, err=map[xmem_909fe0f4039f47383f11bd96b32b87e9/default/default_192.168.1.81:11210_1:dial tcp getsockopt: no route to host]

I have opened the following ports in both Synology routers:

4369 8091 to 8094, 9100 to 9105, 9998, 9999, 11209 to 11211, 11214, 11215, 18091 to 18093, and from 21100 to 21299.

Can someone please tell me what the problem is?