ASUS AX53U
Before blindly doing things it is recommended to read up on the subject.
Going from Stock firmware to OpenWRT is often a risky procedure with a real chance of bricking your router.
See: https://openwrt.org/toh/asus/rt-ax53u
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Read up here:
https://openwrt.org/toh/asus/rt-ax53u#installation_with_mtd-write
If I understand the documentation page correctly, using the Web-UI of your router is currently not possible.
Although you didn't provide much text to work with, your screenshot is just enough to determine you're doing it wrong.
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Great it is working. 
If your problem is solved, please consider marking this topic as [Solved]. See How to mark a topic as [Solved] for a short how-to.
Thanks! 
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