RainOne 101 router is useless. Lack basic function of changing IP Address

Their 5G router RainOne 101 is useless, does not have the basic function of changing internal IP address (gateway/DHCP address) and when you call them, they say this is not allowed, which means, in order to use this as a backup, I will need to change all the IP address settings within the network......and we in 2024!