I'm following the Multi-DC Consul on Kubernetes to implement the example. Everything works perfectly fine during the configuration. After the configuration, consul members of two data centers are as below:

Amsterdam:

Node                                Address           Status  Type    Build  Protocol  DC   Segment
consul-0                            10.20.2.6:8301    alive   server  1.2.1  2         ams  <all>
consul-1                            10.20.0.5:8301    alive   server  1.2.1  2         ams  <all>
consul-2                            10.20.1.11:8301   alive   server  1.2.1  2         ams  <all>
gke-ams-default-pool-a85b4aae-1gtl  10.154.0.14:8301  alive   client  1.2.1  2         ams  <default>
gke-ams-default-pool-a85b4aae-kktf  10.154.0.15:8301  alive   client  1.2.1  2         ams  <default>
gke-ams-default-pool-a85b4aae-m4dj  10.154.0.16:8301  alive   client  1.2.1  2         ams  <default>

London:

Node                                   Address           Status  Type    Build  Protocol  DC      Segment
consul-0                               10.28.1.8:8301    alive   server  1.2.1  2         london  <all>
consul-1                               10.28.0.9:8301    alive   server  1.2.1  2         london  <all>
consul-2                               10.28.2.14:8301   alive   server  1.2.1  2         london  <all>
gke-london-default-pool-1ea5feef-0t50  10.164.0.22:8301  alive   client  1.2.1  2         london  <default>
gke-london-default-pool-1ea5feef-329g  10.164.0.21:8301  alive   client  1.2.1  2         london  <default>
gke-london-default-pool-1ea5feef-z62n  10.164.0.20:8301  alive   client  1.2.1  2         london  <default>

Then I tried to join services in Amsterdam from London and run consul join -wan 10.20.2.6. After a while, I got the error

Error joining address '10.20.2.6': Unexpected response code: 500 (1 error(s) occurred:

* Failed to join 10.20.2.6: dial tcp 10.20.2.6:8302: i/o timeout)
Failed to join any nodes.

What could be the possible reason for this error?