Login to your selfcare via pc first, then click on charges. You should see list of bill statement which once you click on your choice, it should open pdf format. Once open, you can then print it out from your end
You can go towards https://mobile.unifi.com.my/selfcare/charges and there should be "View/Print" next to your bill charges amount. Click on it and there will be a notification of "Kindly ensure your popup blocker is disabled for our site."
PS: If you did not make payment via AutoPay, the other "View/Print" function will be your official payment receipt.