How can I downgrade iOS 26 beta to iOS 18.5 without a computer?

You absolutely need a computer.

Borrow one from a friend or relative or take the device to an Apple Store and see if they will let you use a floor mode to restore your device.

You won't be able to return to iOS 18 without a computer.

All we can do here is point you to the uninstall instructions. When installing a Beta, you are entirely on your own. Apple does not offer any support for Betas, and as such we are not allowed to offer any either in these forums beyond posting the link to the uninstall instructions.

click here ➜ How to uninstall iOS or iPadOS beta software - Apple Support

You will need to place the iPhone in recovery mode as explained in the link above.

You can try making an appointment at your local Apple Store and seeing if they will help you.

Genius Bar Reservation and Apple Support Options - Apple

If you don't have a computer, you likely didn't read any of the documentation provided when you installed the beta. Had you done so, you'd have known you needed to create an Archive Backup on your computer first. Since you likely didn't do that, when you do get access to a computer, your phone is going to be wiped in the process. This means you're going to lose all the data on the phone.

You're going to learn a valuable and painful lesson here.

You’ll need a computer. Probably the one that you use as an app developer.

How to uninstall iOS or iPadOS beta software - Apple Support

First tell us how you managed to install iOS 26 since it has not been released to the public.

All the relevant information was provided to you when you enrolled in the developers beta and accepted the Non Disclosure agreement.

READ the information that was provided to you.