Are you using thread or wi-fi? Do you have an Ikea Dirigera hub?
My encode plus has been on thread the past year (with incredible battery savings compared to wi-fi before that, BTW) but it uses a nearby HomePod mini to connect to thread and is happily online right now. Having said that, I've also had an Ikea Dirigera hub online for about a year and - in the last few days - I've been reading online that a Dirigera update and/or the Ikea Homesmart app may be the cause of thread/matter devices dropping offline recently.
Personally, I've been trying to integrate a bunch of the new Ikea matter devices into Apple Home the last few weeks and it has not been plain sailing - so there may be something in it?