It is at least possible that Roon’s requirements grew during that time and an underspecced machine that was barely sufficient a year ago is not sufficient now.
I would recommend following @Rugby’s advice to install Roon server instead. The GUI does not work anyway on this machine and, if I understand you correctly, never has. So you are not losing anything and the Roon Server will most likely work faster as it’s fighting less for resources on the underspecced machine.
However, it’s also possible, maybe likely, that the iPad connection error is completely unrelated. Did you change anything in your network setup or on the PC or iPad?
No, never. IP addresses in a network must be unique. They are literally addresses, and if two devices have the same one, essentially the same happens as with postal mail when two houses have the same address.
Maybe you misunderstood and the FAQ said that that the IP addresses must be on the same network. This is true. There are several networking addressing schemes but home networks usually use IP addresses of class C in the private range 192.168.x.y. In this case (and except in pathological configurations), x is a local subnet and y is the individual machine. So 192.168.1.y is in one subnet, and 192.168.2.y is in another subnet. On the same subnet, like 192.168.1.y, one machine could have the address 192.168.1.1 and another 192.168.1.2 (These machine numbers can go from 1 to 254, so you can have 254 machines in a subnet when using class C addressing. 192.168.x.0 and 192.168.x.255 are special, they are the network address and the network broadcast address, respectively).
Data generally cannot traverse across subnets without special configuration on the router, and Roon does not traverse across subnets either, so all Roon machines that need to talk to each other must be in the same one.
Normally, the router takes care of unique addresses automatically with its DHCP service, and usually they will come from the same subnet. Essentially, when a device comes online, it sends a DHCP request to the router, asking for an IP address, and the router will send it one that is not used by any other device. Unless someone disables DHCP or assigns static addresses on the devices themselves (which is a really bad idea in most home networking situations), this is usually nothing to worry about.
However, some router settings can create different subnets for the wired ethernet and the wifi. In this case, an iPad on the wifi would not be able to talk to a core on the ethernet. So to rule this out, check your devices’ IP addresses with special focus on the subnet part