I suffer from double nat I would like your help

I will explain how my network is

I have my ISP router in bridge mode where it is connected to openwrt and connected to a tp link ax20 router that distributes internet by cable and wifi

ISP > OpenWrt > tp-link Wifi

here the problem, internet is slow, i can't download anything and i also don't watch videos it is too slow but if i put my tp link router in AP mode internet works great then i read that it is due to NAT so i thought disable the NAT option would solve my problem but it was not like that

You need at least one router doing NAT, what you do not want are two routers doing NAT. Now, it putting the second router in AP mode works for you, what is the problem now?