Homekit does not work remotely for control, but does work on local wifi.

Recently, my apple home / home kit stopped working properly. I can no longer control homekit accessories remotely.

I am able to control accessories fine when I am on the same wifi network, but as soon as I go onto cellular they stop working. I can also see the status of all my accessories, whether i am on the same wifi, or remote. It is just control that i seem unable to do. I am able to control the accessories via their native app remotely, just not via home app. This applied to all accessories i own. This series of debugging leads me to believe there is some problem with how my home hub is communicating with apple, or how my iphone is communicating with apple.

I have tried the following steps to resolve it:

• restart router, modem and home hub (apple tv), restart iphone
• logged home hub out of icloud account
• removed hub and readded
• removed accessory and readded it
• ensured same icloud account is used for iphone and home hub
• ensured "control accessories remotely" is on in home app
• ensured icloud data is saved toggled on for home
• ensured iphone mobile data is enabled for home.
• ensured apple tv and iphone are both running latest tvOS/ios 26 respectively.
• Opted out of CGNAT with my ISP.
• Asked if my ISP knew of any known issues related to this, they didnt.

I am out of ideas what might else have caused it to go wrong. The only two things i can think of are:

• the issue seemed to coincide with when i changed ISP.

Are there any other things i should check?