How to organize access to the server through two WANs simultaneously using Zyxel Omni2 with NDMS2.09?

Question

Computer networks

The initial task - to make sure that the server application running on one of the computers on the local network could receive requests from the Internet to its TCPxxx and send back responses, was perfectly solved using a white IP and the first router that came across that could NAT and port mapping.
Everything worked well for several years, but then a person responsible for generating ideas appeared in the office and it began: -
We need Wi-Fi! - Ok, here is a router with Wi-Fi.
-And now we need a spare channel! - ok, here is a router with wifi and usb for lte modem. And a modem. And failover from the wire to 4G.
-And now you need the program to work on the spare channel, but our operator does not give a white IP to the modem! - ok, here is the second provider with a cable and a white IP, but a router with support for several ISPs and with usb for the modem, you still want a modem later.
-A... And now you need to be able to connect to the program using any of the IPs, regardless of which channel is currently working!!!
And with my head, I understand that Zuhel with this new firmware of theirs can do this, but I don’t know how.
On the forums, a thread with a subject question usually ends with words like: "... having suffered a lot, I also set up simultaneous access. Somehow I'll tell you later." or "... but making the responses go through the interface through which the request came is much more difficult."
I did not find anything similar in articles about typical solutions on zyxel.ru and help.keenetic.net (or maybe they just hid it well).