11 November 2021
You have to calculate tax on advance and pay tax while filing the return for the month. The advance received should be grossed up. This means that the advance received is considered inclusive of GST. Issue tax invoice for the advance received and show it in GSTR 1.
12 November 2021
No need to file IFF for advance. You will have to calculate your liability for the month though and pay through challan by 25th November 2021