Can I invest every month as a lump sum instead of SIP by keeping track of the NAV?

What you are planning to do here will be beneficial theoretically but practically, it is not possible to invest exactly when the NAV is least in the month. This is because nothing is certain in mutual funds and the NAV can fall even further from the point that you think is the least. However, if you can invest a lump sum amount every time when the NAV is least for the month, you can gain better returns as more number of units will be accumulated. In SIP, a pre-decided amount is invested on the fixed date of every month whether the NAV is high or low hence, the risk is averaged out and discipline is maintained. With manual SIP or lump sum investment every month, one might miss the monthly investment or can end up investing at the wrong time hence it is a high-risk move.