Need help with IP address authentication when logging in

I am from South Africa and we are experiencing regular load shedding when power is off for at least 2 hours. This results in routers and modems resetting and then new IP's gets allocated. The result is that users trying to connect cannot due to new IP. What we need to do is to allocate a range of IP addresses to cover that. A typical address will be like: 105.224.69.68 and the new IP allocated might be like this: 105.184.212.163. Only the first section "105" is constant (depending on the ISP.) Access currently is only allowed if the username, password and IP address match.

We need to specify a range of IP.s to cover that so if he logs in with 105. whatever follows he will be able to log in with his username and password. Any suggestions?

That's just not possible. There are 2^24 (17 million) possible addresses in your "range".

Thank you, any other ideas to overcome this issue. Our national power supplier with regular load shedding is creating havoc, need to constantly then changing user IP addresses to log in

Use dynamic DNS:
https://en.wikipedia.org/wiki/Dynamic_DNS