The remote sensor inside your TV is not working. It does not receiving infrared signal generated by your remote control. There are two main reasons for this problem. The first one is the defective infrarred sensor itself. The second one is, there is not sufficiant voltage to the remote sensor to work properly. The remote sensor can be located just behind the infrared sensor window at the front of the Tv. It will be diretly soldered to the front panel control printed circuit board. It is a thhree terminal component, look brownish black in colour, very cheap to buy, and can be bought from most of the electronic spare part shops at your town. It will look somewhat like this.
There will be typical number printed on its body, and it should be noted when buying a new one. You must ahve a concept of desoldering and soldering, some skill to repair elactronic ciruit boards to replace it by yourself.As far as your sectond complaint is cncerned, lack of brightness, may be of so many reasons. As you know, a TV is composed of hunderes or thousands of small components, and each of them have their own "duty" while the set works. If any one amouhg them have a fault, it will effect the whole working of a TV. Typically, a dark picture may be of ow screen voltage. Screen voltage can be adjusted, and there are adjustment points for screen and sub-brightness. But this is a very skilled work, and and unexpereienced one can't do, and is dangerous. OK.