You can't donate to a charity to gain access to a politician.
I was actually trained on this exact scenario in my last ethics training. The fact scenario specifically said that the charity in our training hypothetical (a hospital) was legitimate. However, that is irrelevant if, for example, the politician in question has ties to the charity. There are circumstances under which it can be legitimate, and there are several cases in which the DOJ declined to prosecute because it found those circumstances to be special, but at a minimum, such a scenario of charitable donations is listed as a "red flag" scenario. There is one of the example fact scenarios used by the SEC.gov site. See the link.
Donations to legitimate charities that end up giving access to benefits provided by a politician are a clear violation of ethics and US anti-bribery law. You are left arguing that this law does not apply to US politicians, but you can't argue that this is rediculous nonsense, and you can't argue that Congress hasn't decided that this type of behavior is ethical...it isn't.
Link: https://www.sec.gov/spotlight/fcpa/fcpa-resource-guide.pdf