Code P0300 troubleshooting question for 2007 Nissan Pathfinder 4.0 V6 113,000 miles new automatic transmission

I have a 2007 Nissan Pathfinder 4.0 V6 113,000 miles with a new automatic transmission put in about 2 months ago . We had gotten the Pathfinder from a family member. They changed spark plugs and not sure what all else they messed with when changing out the plugs not even sure they would even know if ya asked them.🙄 All we know is it was a everyday drivable Pathfinder before they popped the hood and changed the spark plugs and who knows what else and now afterwards the Pathfinder has these present issues here. So when starting up it is rough idling, sounds like crap miss firing  and has absolutely no power at all when trying to drive maybe max speed of 5mph at most on flat grounds now up hill inclines are a diffrent story. We barely was able to make it up our driveway due to its slight uphill. So after we was able to barely making it up our driveway and experiencing the above issues we had noticed with this pathfinder we just left it sit in our yard for awhile trying to figure out what to next to fix this problem. Now when starting it still has all those same issues  but now also wont move at all from where we parked it.  When before at least we were able to barely drive it around. So we plugged up and ran a code reader and its only throwing a singal code P0300. Now after explaining this nightmare with the 07 Nissan Pathfinder my question to you would be what would you recommend we do next to try to fix this nightmare? I look forward to hearing back from you with any advise and help possible.  Thank you and God bless.