IPV4 Static Address reverts to "Obtain an IP address automatically"

Win 10 Pro, 64bit
Ver 21H1



We have a network that has five computers, an Ethernet attached printer, an EFTPOS terminal and a Wifi enabled SD card connected. Each device has a fixed IP and we use a hosts file in windows C:\Windows\System32\drivers\etc
This might seem an odd way to do it, but it is a necessary one because of the way the EFTPOS and Wifi SD card operate - they both have fixed IP numbers that we cannot change. I will explain about this at the end if anyone is interested.
The problem we are having is that one of the computers reverts to "Obtain an IP address automatically" on every reboot. This is the procedure I follow, which I hope is correct...

Control Panel > Network and Sharing Centre > Change Adaptor Settings
Right-click on the Adaptor and choose Properties
Left-click on Internet Protocol Version 4 (TCP/IPV4)
Click the Use the following IP address radio button and input
IP Address 192.168.1.99
Subnet Mask 255.255.255.0
Default Gateway 192.168.1.1
Check the box marked Validate setting upon exit
Click OK and Close
Check to see if it has worked by going to any other computer on the network and using an Admin Command Prompt to run the command arp -a which lists all the IP numbers in use on the network. "99" is there!
Then I reboot the computer computer in question, and check arp -a again, and "99" has gone,replaced with some other number. When I check the TCP/IPV4 settings, it has reverted to "Obtain an IP address automatically". I cannot get the static IP number to "stick". Is there anything I am doing wrong here - am I missing some obvious or non-obvious vital step?

******
Why our network is configured the way it is.
The Wifi card is in our shop camera, and it only comes on line when some photos have been taken and the card wants to send them. It connects to 192.168.1.65 by default. After a few minutes, the camera turns off and the card disconnects. Similarly, the EFTPOS terminal (192.168.1.20) only connects to the network when its Logging On, putting through a transaction or running the EFTPOS Settlement at the end of the day.
If we allow our computers to "Obtain an IP address automatically", sooner or later, one of them will steal the Wifi Card or EFTPOS terminal fixed IP, and that device won't work.

You can do it the other way most routers even running dhcp on the router you can tell it to always give a pc a certain ip. When you set ip from cmd prompt type
Ipconfig /all

Post results reboot and run again post results.
It could be the ip is in use or reserved

smartcooky said:

.The Wifi card is in our shop camera .... It connects to 192.168.1.65 by default. ..., the EFTPOS terminal (192.168.1.20)...

...If we allow our computers to "Obtain an IP address automatically", sooner or later, one of them will steal the Wifi Card or EFTPOS terminal fixed IP, and that device won't work.

Almost every router I've seen, even the free one I got from my ISP, has the ability to set the range of IP addresses in the pool it allocates from. The simplest solution would be to set your router's DHCP server address range of IP addresses to start at 192.168.1.66. Then no PC can ever get assigned either of those two critical IP addresses. This also means you can forget about assigning static IP addresses to every PC, one setting on your router should fix it for all current and future PCs on your network.

Here's my router's control panel as an example....