I hate the 7i97T

Input 8  (TB5 pin 1)  is

hm2_7i97.0.inmux.00.input-08
 

This may be obvious to everyone, but just in case: there are two different ways of specifying subnet masks. You can either spell out the mask itself, in either hex or decimal( ff.ff.ff.00 or 255.255.255.0), or you can simply specify the number of 1 bits in the mask: 24. If the mask itself is given, it should always be written as 4 dot-separated octets. If the count is given, it's usually specified with a / character, after the network itself, as in 10.10.10.0/24. (Aside: note that using the count-of-bits method, you can never specify a subnet mask that isn't a contiguous set of 1 bits. Such "non-contiguous" masks are technically legal, but are really hard to manage, so, don't do that.

The default subnet mask for 10.n.n.n IP range is 8 or 255.0.0.0, and it will work with any subnet mask you set, be it 16 or 255.255.0.0 , or 24 or 255.255.255.0
I am sure that was not the issue.
Subnet is used to limit the the access to a certain IP range even when the same starting IP's are used, the thing nobody ever mentions here is:
Non routable IP ranges are :
10.0.0.0 to 10.255.255.255 (10.0.0.0/ so roughly 16 million IP addresses
172.16.0.0 to 172.31.255.255 (172.16.0.0/12) so roughly 1 million IP addresses
192.168.0.0 to 192.168.255.255 (192.168.0.0/16) so roughly 65,000 addresses
Notice the default subnets, they can go up to 32 or 255.255.255.255 but that is useless as that will limit access to 0 (zero) IP's
You can set the subnet to 28 or 255.255.255.240 for the 10.n.n.n range, and still Mesa WILL work, as that will limit the IP block access to only 16 IP's, and since two of those are always reserved, you have 10.10.10.1 up to 10.10.10.14 that will work perfectly fine.
Etc etc, way to many variations there, but in general subnet is not very important ... until nothing works due to an error in it.

Sometimes though the subnet mask does become important. I was allocated a 64 IP address range from my ISP way back when it was a dedicated dial up connection. Then the mask was important /26 or 255.255.255.192. But when setting up your own networks, just using 255 IP address are plenty and if you need seperate subnets, use the 192.168,  172.16 and 10 ranges or you can use 192.168.1.x, 192.168.2.x etc. You might need 2 seperate networks if you have a router with 2 WAN ports so you can have a failover connection or DMZ.