My LO is 6 months and doesn't sit unsupported yet!
What you can do is put a pillow underneath the top half of their body, enough to elevate them. You might need to roll up a blanket to elevate them a bit and put it under the pillow. Doing some foot sensory at the same time like a foot piano. So they start getting interested in propping themselves up, and seeing what they’re doing. I found this helps with their core strength. It takes practice but it helped me to sit up. Also, using breastfeeding pillow seat things to help support them. My baby’s still not fully rolling back to front. So dw every baby develops in there own time try doing some exercises x
The ‘norm’ for rolling can be any time from 4 to 8 months so don’t worry yet 😘 from experience with my 2 boys, both of them have rolled one way for a couple of weeks then started the other day quite soon after x