1m thick of what? Concrete!!! Is it an old bunker? If that is the case then your client is being unreasonable to expect a wireless signal to be sufficient and their opposition to running ethernet cable to site is headstrong and illogical. As others have stated, an ethernet cable in some small trunking/conduit buried is the only way to ensure a reliable connection in what appears to be evolving circumstances. I have run cable inside and outside of listed buildings in some of the most difficult circumstances so maybe there needs to be a compromise with your client.