Right... so that means that you cannot use it an an entry point for ssh (or any other services). That is why you are getting the error... when you attempt to ssh to your system via that IP address, it's not actually able to get to your machine because that IP isn't exclusively yours. You would need either a port-forward or an exclusive IP -- both of these things are not standard for most commercial VPN services.
As an outbound IP, yes... but not as an inbound one. This further proves that what you are attempting to do will not work. This is not related to OpenWrt.
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Thanks! 