Ok, looks like I've found another free alternative https://exchangerate.host/ if you want to try this. You'll need to define a new Exchange Rate Service like so

public class ExchangerateHostCurrencyExchangeRateService: CurrencyExchangeRateServiceBase
{
    private const string API_BASE_URL = "https://api.exchangerate.host";

    public override decimal FetchExchangeRate(string fromCurrencyIsoCode, string toCurrencyIsoCode, DateTime date)
    {
        return Get<ExchangerateHostSingleResponse>($"{API_BASE_URL}/{date:yyyy-MM-dd}?base={fromCurrencyIsoCode.ToUpper()}&symbols={toCurrencyIsoCode.ToUpper()}")
            .Rates?.First().Value ?? 0;
    }

    public override Dictionary<string, Dictionary<string, decimal>> FetchExchangeRates(string fromCurrencyIsoCode, string[] toCurrencyIsoCodes, DateTime dateFrom, DateTime dateTo)
    {
        var results = new Dictionary<string, Dictionary<string, decimal>>();

        LoopByMaxDateRange(dateFrom, dateTo, 365, (startDate, endDate) =>
        {
            var r = Get<ExchangerateHostTimeframeResponse>($"{API_BASE_URL}/timeseries?base={fromCurrencyIsoCode.ToUpper()}&symbols={string.Join(",", toCurrencyIsoCodes.Select(x => x.ToUpper()))}&start_date={startDate:yyyy-MM-dd}&end_date={endDate:yyyy-MM-dd}");

            if (r?.Rates != null)
            {
                foreach (var rate in r.Rates)
                {
                    results.Add(rate.Key, rate.Value);
                }
            }

        });

        return results;
    }
}

internal class ExchangerateHostResponse
{
    [JsonProperty(PropertyName = "base")]
    public string Base { get; set; }

    [JsonProperty(PropertyName = "date")]
    public DateTime Date { get; set; }
}

internal class ExchangerateHostSingleResponse : ExchangerateHostResponse
{
    [JsonProperty(PropertyName = "rates")]
    public Dictionary<string, decimal> Rates { get; set; }
}

internal class ExchangerateHostTimeframeResponse : ExchangerateHostResponse
{
    [JsonProperty(PropertyName = "rates")]
    public Dictionary<string, Dictionary<string, decimal>> Rates { get; set; }
}

And then in your Startup / Composer you'll need to swap out the default implementation like so

builder.Services.AddUnique<ICurrencyExchangeRateService, ExchangerateHostCurrencyExchangeRateService>();

I'm going to change this to be the new default, but if you want on going stability, you might want to look at one of the paid / registered options.

Hope this helps

Matt