Can your baby lift up his/her neck without support? If yes, then you can prop some towels beside at the side to help support topping over, but if baby neck is not able to hold by them self, I would not advise you to feed baby food as it might cause choking.
you can google youtube videos on tips how to train baby to sit. such as V position or tripod. give baby time too and not rush them according to guidelines. mine only sat better at 6.5-7 months.
every baby different my daughter was able to sit stable 5 months plus my son going to be 6 months still very wobbily cant sit yet. Doc say normal