WILL THIS £20 LUBE FIX MY £650 L322 RANGE ROVER?

After doing a gearbox service, my £650 L322 Range Rover is still having issues. So here it goes, will a £20 tube of "Dr. Tranny Instant Shudder Fixx" sort my gearbox out? I also find more issues that could be causing the problem.
Here's some of the tools I used:
Lubegard 19610 Dr. Tranny Instant Shudder Fixx 2 Ounce: amzn.to/3XOgz4X
3L Vacuum Pump Tool: amzn.to/3TIXoYX
Torx & Hex Set: amzn.to/4gA7XYh
Fuchs Titan ATF 4000 High Performance Automatic Transmission Fluid 5 Litres 5L (Please check this is the right fluid, lots of debate out there) (Not affiliate): www.ebay.co.uk...
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[@cjparr]

16MPG. Is that with you trying to be economical, or do you put your foot down a lot?

[@JAYPCARS]

No matter what I do, I can’t seem to get it above that. Town and B-road driving mainly 😦