YES it works. I also bought the RT-AX89X a few days ago and connected it to the 10 G from Init7 and it works! :-)
Greetings

I now have Init7 with 10Gbit and the router ran immediately without problems. I had Init7 send me the SFP+ module and I use the fibre optic cable article number 5679650.

Have you found the answer yet? I'm also wondering if the router works with Init7. WAN via SFP+ seems to work.

Yes, it works. Just note that you need the appropriate SFP+ module, but you can order it directly from Init7. If you still need a recommendation code: 60602764819

I don't know about Init7, but it should work without problems. I use Salt and that without problems.