There's a section on brightness control on page 132 of the user manual here: https://downloadcenter.samsung.com/content/UM/202109/20210910124253866/OSNDVBEUA-7.2.0_EM_OSCAR_EU_E...
In that case, it might be worth a quick call to our TV/AV specialists, who'll be able to make a remote connection to your TV to check over your settings. You can call them on 0333 000 0333 until 6pm this evening, or between 10am to 8pm tomorrow. Hope this helps!
Have a fantastic New Year!