I have signed up for AutoPay using bank account. Will I still get autopay discount of $5 per line, if I make one-time payment of part of my bill using a credit card, when bill comes

It is super important to understand different discounts because no one wants to miss out on any kind of savings. Check out AutoPay on T-Mobile Support to get more details. 

• Be on Go5G, Magenta, T-Mobile Essentials, ONE Plan, or another tax inclusive plan (such as LineLink and Mobile Internet for hotspots).
• Be active on AutoPay at the time of bill processing.
• Have paid the prior month's bill by its due date.
• Cards with statements mailed to foreign addresses can’t be used for AutoPay.
• Payment method must be a debit card or linked bank account. Credit cards and digital wallets (Apply Pay/Google Pay) are not eligible.

The last bullet point there means that the payment method on the Autopay has to be one of the eligible types. You can make additional payments earlier in the bill cycle with a credit card and still get your autopay discount if your payment method there is the debit card. When you are making that partial payment, you will want to make sure that the Update Autopay box is NOT checked, so you don’t risk issues there.