ADSL + WAN instead of 3G modem on Asus DSL-N14U router, is it possible?

Question

Computer networks

There is an Asus DSL-N14U router, a simple one. it is not a more recent version of B1. It has ADSL, 4 LAN ports and USB for flash drives, printers and 3G modems, but no WAN port, unfortunately. Now it works via ADSL and it is, at the moment, a slow but stable Internet at a speed of 6 receive / 1 return, but now it is possible to use WAN from another router that works via fiber, there are speeds of 100/50, but there is nowhere to plug it, except directly into the PC. Is it possible to make the router work in Dual WAN mode, despite the fact that it is not there initially, or plug the WAN through the LAN to USB adapter and connect it all to the USB connector on the router, can such a bundle be made to work? The main problem is that I have RT-AC66U at home, which receives WiFi from DSL-N14U and duplicates it,
I see three options:
1. Make the DSL-N14U router with ADSL + WAN work (like Multiwan, but I’ll never know how to set it all up, for TP-Link, D-Link and other routers I saw a bunch of manuals, but for Asus, damn it...) and don't touch anything else - the RT-AC66U router will remain in its place as a repeater
2. Make the DSL-N14U router work with ADSL + WAN connected via USB yo LAN adapter type WAN instead of a 3G modem, but I don't quite understand if it's possible, even in theory, googling didn't give much results either :|
3. Pair RT-AC66U with a Zyxel ADSL router and turn on Dual WAN to configure ADSL over Ethernet + connect WAN to one of the LAN ports and reassign it to WAN, and use DSL-N14U as a repeater
Thanks in advance to all who responded!
PS The funny thing is that ASUS claims to be able to connect via Ethernet, ADSL and 3G / 4G in order to be able to use a backup network or to increase network bandwidth, quote:
Total flexibility with DSL, Ethernet or 3G / 4G LTE internet connections
DSL-N14U has multiple ways to connect to the internet, with a choice of DSL, Ethernet or 3G/4G LTE*. You can use DSL-N14U as a DSL modem Wi-Fi router or a standalone Wi-Fi router according to your needs. And with the ability to switch instantly between connections if one fails (and switch back again when it's fixed) or combine multiple connections for more bandwidth, it's perfect for reliable, always-on internet connectivity.

Answer 1

but there is nowhere to plug it, except directly into the PC.

What about an adsl router?
Damn me, can you at least write what you want to get?
If you draw a map, it will be great.

Answer 2

you will make an ordinary router from ADSL only by completely rewriting its firmware)
Two routers will solve your problems - adsl to your current one, cable from it to Mikrotik, second incoming link to Mikrotik and already balance the channels with Mikrotik

Answer 3

The issue is resolved, in the comments I described the solution to the problem