All error codes

vpn_not_enabled

HTTP 409 The VPN tunnel is not enabled on this router

What happened

The tunnel is switched off or was never switched on, so there is nothing to add a device to.

What to do

Send PUT /v1/servers/{id}/router/wireguard or /router/openvpn with enabled: true, then repeat the request with the same idempotency key.

Retrying

Change the state this refers to, then repeat the same request with the same idempotency key.

How it arrives

{
    "type": "https://docs.fiberax.com/errors/vpn_not_enabled",
    "title": "The VPN tunnel is not enabled on this router",
    "status": 409,
    "code": "vpn_not_enabled",
    "request_id": "a277dcd1ca652c65"
}

Branch your code on code, never on the wording of title.