You need (fanfare)
Your private (IPv4) network MUST use one of the ranges allocated by this RFC:
10.0.0.0 - 10.255.255.255 (10/8 prefix)
172.16.0.0 - 172.31.255.255 (172.16/12 prefix)
192.168.0.0 - 192.168.255.255 (192.168/16 prefix)
Anything else is Just Plain Wrong. And you can tell them the people who built the internet said so.
OK, so there's really no
MUST anywhere in that RFC, but in practice the internet shuns heretics who just grab random swaths of IP space for their private networks, and everyone uses the RFC-1918 reserved ranges for their designated purpose of private networks. Do it right and fix your configuration to use standard, RFC-confomant addresses for your private network, and I bet your funky unreachable devices issue will go away...