No, I don't think they're referring to the heat. I presume they are referring to the celluar service/signal by "modem". The 6 series phones have lots of complaints about that aspect. In fact it's (personally) my only complaint. It's not unbearable for me like I see in other random internet peoples complaints, but it is noticeably worse than any other phone I've had in recent memory. I live in a major metropolitan city, I can't imagine how bad it might be in a rural area. That said your mileage will likely vary based on your location and luck of the draw.

My warning would be to make sure you get it from a reputable retailer where you have the opportunity to return it if it doesn't work out.