New Investor Looking To Learn

Hi All, my name is Adam and I am from Concord, NH. In 2020 I purchased a small condo and have recently moved out and am renting the unit out for a nice return. I am looking to add more inventory intelligently and cautiously as I am in no rush. My thoughts are in 2 stages currently. Should I sell my condo and use the revenue to purchase more units or should I use a HEloc/HEloan. The revenue from a sale or from a heloc would leave me with roughly the same amount of cash on hand, close to 100k. The next thought is whether I should or even could take that and invest in 1 or 2 properties with this 100k. 

My initial planning and research is to get a better bead on the rental market in my area, understand the market for duplex/triplex properties, and plan through the best way to add a new unit in my area. Thanks all!