I used to handle probations in a corporate environment.
Did you tell them you were dyslexic at interview/ acceptance?
Firstly, your probation was not extended after finding out your were dyslexic, it was extended when it was discovered you were unable to perform a key duty. You disclosed your dyslexia at that point, so the company are now obliged to make reasonable adjustments to allow you to do the role.These adjustments will be made and providing you're able to perform to the required standard with the adjustment in place, you'll be passed at your next probation review.
You're pissed off your probation has been extended which you're entitled to feel, but your attitude is all wrong here and that's your main problem. Don't waste your energy looking for discrimination and feeling like a victim, instead think about ways they could help you do your job in light of your disability and communicate that to your line manager/ HR.
Your tone in your OP is hovering dangerously close to 'I'll just quit because I've been discriminated against' mentality here and it's not even half time yet. There's every chance they'll pass your probation once the adjustments have been made and it will all be fine. If you're still unable to do the job after the adjustments, then you're not right for the role and it's better for all involved that you find a different job.
Sorry i that was a bit blunt OP, it's a shit situation but your company aren't doing anything wrong here.