First understand the basic thing - Credit card bill repayment works on banks internal network via NEFT/RTGS/UPI/IMPS. There is no relation between card network and credit card bill repayment.
Union bank is PSU bank, they dnt integrate credit card bill repayment properly.
Pay same bank RuPay credit cards bill and you will see your bill pay will update instantly or within 3 days (depend on your bank)