WiFi Hotspot is okay, but how to make a wireless router?

[jitagaki]

I've been successfully using your command to make wireless hotspots, first with my low spec hp "desktop" (basically a laptop motherboard stuffed in an empty desktop case powered by a laptop power supply) Puppy Linux/Ubuntu machine, and now running on a headless Raspberry Pi 3B+ running Raspberry OS lite.
I moved the desktop to a different room and could not reliably connect wirelessly to the hotspot. I believe the desktop's onboard network card is flakey, at least in the 2.4 GHz band. Not wanting to run a cable I decided to connect by using an old Linksys WRT54g router running dd-wrt in Client Mode, basically as a glorified wireless card.
After many failed attempts, it occurred to me that the wireless hotspot is an access point but probably not a router and Client Mode is for router to router.
So, (sorry for the long-windedness) how do I setup a wireless router using your command? I tried:

sudo /home/linux-router-master/lnxrouter -i wlan0 <my ssid> -p <my password> -o eth0 &

in my /etc/rc.local but no dice (obviously!).
Is it possible through your command or do I need to power-up my CLI-fu?
I want :
Internet --- cable modem ---(eth0) --- RPi3+ --- (wlan0) router -- dd-wrt router in Client Mode -- (eth1?) -- desktop client

(There are other wireless clients on the wlan0 router but I couldn't figure out the linefeed tab whitespaces :( !! )

Thanks.