I have problems playing him because he dies too fast. When he goes to the front he just get sliced by General Grievous and / or Count Dooku and can not really attack and as a token generator it is much easier to play clones with electro binoculars for the aim. How should I play him so he is worth his points?

Do you play Obi-wan, also? Or just Rex as your Commander?

Rex by himself should never run ahead of your troops. If you're running Obi-wan, Dooku and Grievous would target him, instead of Rex. If you're playing Rex by himself, he should stay within your Clone's fire line or death ball. If Dooku and Grievous can charge him, then they should also be able to get shot by your corps.

Use Take That Clankers more as a means to give Rex range 3, so he can attack more safely for an important turn. Call Me Captain is incredible, where you can Fire Support every corps unit you have, if positioned well.

Generally, it sounds like you're being too aggressive with him, where instead you should rely on him to deal damage mid- to late-game. He's not a first strike piece, but by the time your clones get where they need to be for the objectives, Rex should be able to fire at some targets. He is great as a token generator in the early game, better than binocs because he can generate an aim just by moving, and then could dodge, or otherwise, and that does not take actions away from your corps units (which binocs' Spotter 1 does), which should be the main source of damage in a Rex-only list.

Rex, on paper, looks like a hard hitting and pretty survivable unit. In reality, he melts if you put him in sight of basically any amount of enemy forces. He's entirely there to provide support for your troops in the form of his command cards and lots and lots of aim or standby tokens. Once your opponents force has been taken out substantially you can jump out with him into range of a lot of enemy forces and play Call Me Captain maybe round 4 or 5 just to wrap up the game, but i do this only if its completely safe for him because he will die sooo fast if your opponent has much firepower left.

Also I would advise him always to be run with Obi-Wan. Obi-Wan can guardian any shots that get to him making him significantly more survivable. When used well he can be worth so much more than 90 points but its also very easy to waste him completely.