1. If the external IP address of your server changed when you moved servers then you'll need to log in to your SagePay account and add the new IP address to the list of valid IPs: https://live.sagepay.com/mysagepay/settings.msp (if the external IP didn't change then it can't be that).
2. When did you first get the error? I ask because SagePay stopped support for TLS1.0 and TLS1.1 yesterday (13/6/2018). This is in response to the changing PCI DSS regulations but SagePay have gone a step further and are only allowing connections using TLS1.2.
If your plugin was written for nop version 3.3 it's very unlikely to be forcing a TLS1.2 connection. If you've got the source code for the plugin it can be patched. If you don't have the source code you should still be able to fix it by adding this line of code to the Application_Start() method of the global.asax file of the nop website:
ServicePointManager.SecurityProtocol = SecurityProtocolType.Tls12;
If you search the forum for servicepointmanager you'll find plenty of posts about this approach relating to various different payment plugins.
You'll also need to make sure you're running on a version of windows server that supports TLS1.2 and has it enabled. Windows Server 2012 or higher should be fine, 2008 R2 can be patched and enabled, 2008 can theoretically be patched and enabled (though having tried it without success, I ended up moving the site in question to a newer server).