A few things to separate here: (1) requiring electronic payment and (2) charging a convenience fee.
If your current lease says you may pay by personal check or money order, the landlord generally cannot unilaterally change that mid-lease. A renewal is a new lease, though, so if they put it in the renewal you have to decide whether to accept it.
The fee is the more questionable part. If the fee is charged by a third-party processor, not the landlord, and you have a real no-fee option like an ACH or e-check that costs nothing, it may be allowed. But if every payment method through the portal carries a fee, that starts to look like a rent surcharge dressed up as a convenience fee.
For rent-stabilized tenants, adding a mandatory payment fee can be treated as an unlawful rent increase if it is not part of the legal regulated rent. For market-rate tenants, there is less protection, but the lease language still controls. I’d ask them in writing: 'Can you confirm the no-fee payment method available to me under the renewal, and confirm the fee is not collected by the owner?' Also, do not rely on what the managing agent says in an email; check the actual rider before signing. There has been more attention on junk fees in leases lately, but the practical answer is usually to negotiate language giving you a no-fee method.