Flagsmith: has UI and can be self-hosted for free (BSD-3 license).
GO Feature Flag: Completely free, but has no UI, although feature flags can be defined in JSON/YAML files checked into Github
Flagd: Completely free, but has no UI.
Statsig: Free plan covers our needs for a year or two. The .Net SDK is lacking, but we can use REST API instead.
Too expensive/cannot self-host: Growthbook
Kameleoon
Split.io
DevCycle
LaunchDarkly
If that doesn't work, try using a VPN set to the US as country.