I have just been bitten by this on a small business network with an inter-office VPN that needs to route through a separate router/gateway for about a month during a transition to a new ISP.
DHCP option 121 should allow devices to be pushed additional static routes. All Windows, Linux and Apple DHCP clients on the network work as expected but not Android-based devices, and I don't see an option on non-rooted devices to add the static route (not that I really want to do it manually).
It appears someone else ran into this and worked around it by adding routes to the gateway device to route to the new destination on behalf of the client. [1][2] Not sure if that would work in your setup.
Unfortunately the default gateway/router (A Nokia GPON terminator) does not support adding static LAN routes - it's more of a 'fit and forget' home-grade product and isn't too sophisticated (the available documentation's pretty rubbish too!).
The problem will go away in about a month when the Nokia is replaced with a GPON bridge and a Draytek router, then the current 67Mbit VDSL service will be decommissioned just leaving the gigabit FTTP WAN connection.
That's unfortunate. Perhaps one albeit clucky work-around would be to have your Wifi connected to a Linux router, that then routes to your Nokia? By Linux router I just mean a cheap Linux box with ip forwarding enabled and set the routes there? Beyond that I am out of ideas.
Yeah, I started go mentally down the rabbit hole - for example I have an access point running OpenWrt, a few Linux servers and I could find a Raspberry Pi or five, but just decided the gain for the reconfiguration pain was not worth the effort.
DHCP option 121 should allow devices to be pushed additional static routes. All Windows, Linux and Apple DHCP clients on the network work as expected but not Android-based devices, and I don't see an option on non-rooted devices to add the static route (not that I really want to do it manually).