I had the same problem on a Galaxy S10+ SM-G975F/DS that was originally set to the BRI or Taiwan CSC.
I bought a SamKEY credit (1 is enough) and used the SamKEY code reader from samkey.org, version v3.96.7 to change the CSC to BTU (UK and Ireland). The software version is the OXM FUF3 Android 11 update.
Following this, Keys Cafe worked fine. It might reset a bunch of your keyboard settings or so the first time, if you've tinkered, so maybe back up your layouts first. It didn't wipe my data or trip Knox, but you proceed at your own risk. Maybe this is the same problem for your phone. This doesn't root, or require root, on the phone.
Keep in mind that SamKEY credits are available for cheaper than eBay. gsmserver.com seems to sell them for 1.99$, and andtron.com, which is where mine was from, has 3 credits for 5.75$ or so.