Uk number not associated with phone anymore since getting US number, but still want to keep both numbers registered

I have recently come to the US from the UK for a college exchange program and have got a different number. I received a message on with IMessage settings saying that, as my UK number is technically not associated with my iPhone, it will be deleted from my phone in a ‘x’ amount of days. I don’t want this to happen as I want both to be associated with my phone. I have my settings set to receive messages from both my UK, US and my iCloud email but I’m worried my uk number will be wiped from this after the certain amount of days run out, which I don’t want to happen. Will I need to add my UK sim into my phone every 30 days to keep both numbers associated with my iPhone?