Did exactly the same steps as you and it doesn't work. Even the 266 firmware is only for L29 and fails in recovery.after numerous fails, here's how I got 9.1 to install OTA
I have a LYA-L09 (single sim) C432 with software prefix ending 238 after changing regions and all manner of things I gave up.
So I downloaded EMUI9 firmware still on V9 but with a prefix of 266 - a more recent version it seems!
I extracted the firmware to a newly created folder named dload on a usb and connected it by OTG to my mate20 and rebooted the phone
while holding Vol + and - until the logo appears it found the update and started installing
when it rebooted i connected to wifi and right away 9.1 was offered to me (downloading it now)
hope this helps some folk, but check your software version to see if a more recent firmware is available and let you enjoy EMUI9.1
Also pressing Volume + and - does nothing. But nice try.