You need to enter the IP in CIDR format.
Example below:
The /24 postfix is not a port but a representation of how many ones there are in the subnet mask:
CIDR /24 = subnet mask 255.255.255.0 = 11111111.11111111.11111111.00000000
You can count the ones 
After you've configured static IP details with nmtui, you can check with below (dont mind the lo profile for the loopback interface):
$ nmcli -t -f name con show --active | xargs -d '\n' -n 1 nmcli -p -f ipv4.method con show
===============================================================================
Connection profile details (Wired connection 1)
===============================================================================
ipv4.method: manual
-------------------------------------------------------------------------------
===============================================================================
Connection profile details (lo)
===============================================================================
ipv4.method: manual
-------------------------------------------------------------------------------
$ ip -br -4 address show scope global
eth0 UP 10.0.0.7/24
$ ip -4 route
default via 10.0.0.1 dev eth0 proto static metric 100
10.0.0.0/24 dev eth0 proto kernel scope link src 10.0.0.7 metric 100
$ cat /etc/resolv.conf
# Generated by NetworkManager
search home.dehakkelaar.nl
nameserver 10.0.0.1