TPlink C7 v2 USB Printer

From your OP, I believe your printer is connected to owrt device, and the computer that wishes to print is connected to your main router.

What print server package have you installed ? eg. p910nd
What is LAN IP address of main router?
What is LAN IP of wireless client owrt device?
What is WWAN IP of the owrt device?

p910nd print server is very easy to set up.

Have you configured your windows computer connected to your main LAN to print to the WWAN IP of the owrt device? (Raw printing, port 9100)

fwiw, in unlikely event you can't get p910nd working in wireless client mode, I know p910nd works with relayd when I last used it with LEDE 17.

See section 9.10 of more detailed guide for relayd

Update: I can confirm p910nd is working for relayd AND for simple wireless client for 21.02.3 on spare HH5a.

For owrt wireless client only, I had to add a port forward rule for port 9100 (from WWAN) to LAN for printing to function. HH5a seemed to require a reboot for new port forward rule to take effect.

(USB1 laser printer connected to HH5a. The HH5a has default LAN IP WWAN has static IP My Win10 PC on 192.168.10.x networks prints successfully to printer on port 9100.

Earlier, I did try specifying Interface: WWAN, but it didn't seem to work at all with simple 'wireless client'.