Cellular data does not work with wi-fi with no internet

This appears to be an issue at least among Apple iOS devices (but possibly other non-Apple platforms). It has become more of an issue with iOS 18. It is also not an issue for most of the user community because the vast majority have access to the internet via their Wi-Fi network. My terminology for the lack of internet on a Wi-Fi network is "local only Wi-Fi".

We live in an area that does not have internet available except through cellular data. We use a cellular modem (hotspot ... not provided by the iPhone) to access the internet. (The cellular modem data is independent from the cellular data available on our phones.) This presents challenges due to limits on the amount of data available to the cellular modem. To keep the cellular modem data from being used up too quickly, I have limited access to it based on the device identification. Thus, the iPhones do not have access to the cellular modem.

We have a "local only Wi-Fi" network to allow devices to communicate within our house. This admittedly is of rather limited use of a Wi-Fi network. Our printer is on this "local only Wi-Fi" network. In previous iOS versions, we were able to access this "local only Wi-Fi" network and still access the internet through the cellular data on our phones. iOS 18 has changed things to the extent that the cellular data is not available when the iPhone is connected to "local only Wi-Fi". In other words, a "local only Wi-Fi" network and cellular data cannot coexist. (See below for a potential workaround we seem to have found, but this seems rather tenuous.)

Essentially, I think this is a request to the iOS developers to provide a solution to this problem. I am using the user community platform 1) for a possible solution and 2) because I do not know how to submit an iOS performance request. I realize that this issue does not affect the vast majority of iPhone uses, but I feel it is a reasonable request and, since the "local only Wi-Fi" and cellular data could function simultaneously in past iOS versions, it seems like it is easily implemented.

My suggested implementation would be if the iPhone senses that the Wi-Fi network does not have access to the internet, provide a means (via a "on/off" switch) to allow the user to manually confirm that the Wi-Fi network will never have internet access and then revert to cellular data as the only means to internet access.

Below is the solution we stumbled on.

Note that this solution only works if the IP address of the iPhone is static (and known) on the "local only Wi-Fi" network. This is configured in the setup of the local router.

In settings, got to Wi-Fi and connect to the "local only Wi-Fi" network.

Click on the "i" (information) button / link for this Wi-Fi network.

In the IPV4 section, change the "Configure IP" to the Manual selection.

Enter the static IP address that has been assigned to the iPhone.

The subnet mask that i use is 255.255.255.0 and the router IP address is left (or set to?) blanks (empty).

The above "solution" seems like a potentially temporary one that could be broken in future iOS updates, but it appears to work for now.

Thanks for any help and hopefully for consideration in a future iOS update.