Email server behind Comcast modem accessing via cell phones
So I am running a email server internally (and have been for many years). In order to be able to access it internally I run a DNS server with the appropriate internal mappings. This has worked fine on all local devices even laptops that roam in and out of the network. However lately newer cell phones do not appear to be honoring the DNS settings all the time. Typically after a reset they will work but once they leave the network and come back they fail.
On my account I have a single static IP address. The phones when in the network are assigned 10.1.10.x addresses which is the same as the mail server's network. Since my DNS solution no longer seems to work properly (for cell phones only) is there an alternative using the comcast business gateway?
Any help would be greatly appreciated.