You can also repay the credit card of China Merchants Bank through the online banking of Agricultural Bank of China, which is equivalent to ordinary transfer business, but there is a handling fee for cross-bank transfer, and cross-bank transfer may not guarantee the same day. Therefore, it is suggested to use the online banking of ABC to recharge Alipay, and then repay the credit card of China Merchants Bank through Alipay.
As for the "test certificate" you mentioned, I suggest that when logging in to ABC Online Banking, pay attention to whether there is a prompt (light yellow) at the top of the browser window asking for installing security controls. If there is, right-click to install it.