Yes I have actually.
That would explain it perhaps.
I tether off my mobile when at my gf's
This may be the problem, but their email requires the average customer to be clued up with how the mobile industry works. You SHOULD be OK as you are using YOUR phone, but the way the mobile industry works can cause the same IP address to be shared around the general public, and not merely those who live in a certain area covered by a local server, but by anyone from anywhere who happens to be in range of the same subset of masks that your phone connects with when at your GF's place.
It's ironic that this problem has not been accepted by the industry as a standard consequence of "going mobile" given that the latest push has been to get players away from their desktops and accessing their casinos when they are out and about through phones and tablets.
Even with your own home internet connection you can run into problems as IP addresses are usually allocated dynamically, it's just how the internet works, you don't have your own personal IP address, you take one from the pool every time you switch your connection on. It's like hiring a car, and then finding yourself arrested because a previous hirer committed a crime using the vehicle.
If you as a player has little clue as to how things work on the ISP side, you would have little clue on how to explain the same IP address being used by another player. The bigger the player base a casino has, the greater chance there is that an individual IP address gets allocated to more than one player over a period of time.
The safest option is to ignore the marketing that tries to get you to use your casino from a mobile device when out and about, and ONLY use it from your home internet connection, one that is secure and where you share freely with other household members which casinos you all play at to ensure that only one of you has an account at any given casino.
This problem will only be solved once IPv6 is in widespread use, and every device can "own" it's own individual IP address. Implementation seems to be constantly delayed by the ISP industry, even though it would solve the problem of there being such a shortage of IP addresses under IPv4 that dynamic allocation from a pool will end up not being sufficient for the demand, and people will find they can't connect because the pool is empty, and they will be queued for connection.
Other internet services have moved away from using the unreliable pairing of IP address and geolocation and use device specific authentication. The USA has decided that IP address and geolocation simply isn't good enough for online casinos in the legal US market, and they have had to implement other means of identifying the actual physical location of the player's point of connection in order to enforce access to within state boundaries only.