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Component, Admin Tools, Web Application Firewall, Configure WAF. Find the "Enable IP Workarounds" and set to Yes.

Please consult the documentation to understand what happened and why you had to enable this setting.

Contact your host (SiteGround). Point them to this ticket. It appears that they are running a proxy in front of your site which does NOT pass the visitor's real IP address in the HTTP X-Forwarded-For header. As a result, your server only sees the internal IP address 109.199.120.202 as the only source of each and every request to your server, regardless of where it actually originates. Unless they address that server configuration there is nothing you can do.

OK, that makes perfect sense and your additional note is correct, you do understand how their server migration works.

SiteGround moved your site between servers. The old server has IP address "X" and the new server has IP address "Y".

If your site's DNS was managed by SiteGround that's all there is to it. Their server migration process also updates the DNS on SiteGround's nameservers so that your site's A record points to the new IP address.

However, you manage your DNS externally. Incidentally that's also what we do; we use Amazon Route 53 because it's faster to boot and propagates much faster to Google Public DNS, CloudFlare DNS, Quad9, Cisco OpenDNS etc (these are the services most network admins and power user tend to use anyway).

If you manage your own DNS but do not change your A record your traffic hits the old server (IP "X") which acts as a proxy to the new server, i.e. it forwards the traffic to the new server's IP address ("Y"). However, the old server does not set an X-Forwarded-For HTTP header, meaning that all proxied traffic appears to be coming from a visitor with the IP address of your old server ("X") which is what causes the problem. This is the point where I strongly disagree with SiteGround's handling of the migration. Since the old server is now set up as a proxy it should definitely set an X-Forwarded-For HTTP header. What they're doing right now affects very few people, like us, but it's really annoying and disrupting :/

Since you've changed your DNS A record to point to the new server's IP address ("Y") you can and must disable the Enable IP Workarounds settings. The way SiteGround servers are set up you don't need it and leaving it enabled when you don't need it can open you to certain types of attack.