SEP-IRA custodian refuses to record April 2023 contribution for 2022--how to handle so 2023 contribution is not excess? IRS rep said custodian is wrong but won't budge.

I spent 30 minutes on the phone with the IRS who told me to point Fidelity to publication 560 - still they refuse to correctly record a contribution made by April 15 for previous year in SEP-IRA.