Backdoor Roth contribution without a U.S. residential address – tried Fidelity and Schwab, any alternatives?

I’m a U.S. citizen living overseas. In the U.S., I only maintain a rented mailbox (commercial address) for receiving mail, and I don’t have any relatives’ residential address I can use.

I want to open both a Traditional IRA and a Roth IRA, and use the Backdoor Roth method to contribute my foreign income into a Roth IRA for retirement investing.

Here’s the problem I ran into:

• I first tried Fidelity, but shortly after opening the account, my transactions were restricted because my U.S. address was flagged as a commercial address. Fidelity required me to provide a U.S. residential address.
• I then opened both a Traditional IRA and a Roth IRA with Charles Schwab International (since I don’t have a U.S. residential address, I could only qualify for the International account). However, with this setup, I can’t complete a Backdoor Roth conversion online and instead have to submit paper forms, which is very inconvenient. The customer service rep suggested this is because it’s an International account.

My question: Are there any other brokerages or banks that allow U.S. citizens without a U.S. residential address to make Backdoor Roth contributions? Thanks!