If that decision was the basis of a MR then your next step is tribunal. Get dome advice & help because a tribunal can award down as well as up.
Look up fightback4justice on Facebook they did all my paperwork no win no fee & if you win you pay them their fee put if your back pay so there’s no real financial hit that way. They’re superb people write superb papers & will phone talk you thru everything too.
You’ll wait 8-12 months for a tribunal & if they increase the award that figure will be backdated to your last award notice.
If you lose a tribunal you can take it to a higher tribunal only on a point of law, ie those before made a mistake in law.
Hope you get it sorted, I had to go to tribunal, thanks to paperwork done they found in my favour inside 3 minutes everyone there Inc security were lovely lovely people too & I was literally shaking I was terrified.