Your router is Linux based. There are two options for an OpenWRT print server; CUPs and p910nd. p910 has a smaller footprint and is easier. If your are using cups, you can download a *ppd for your particular printer at OpenPrinting
https://www.openprinting.org/printer/Brother/Brother-HL-5040
There are multiple ways to do this.
The HL-5040 is PCL5e/PCL6 capable. Per the Openprinting web site, the PCL5e driver is fast and reliable but only goes to 600dpi. When a print job is requested from most applications, it is generated in Postscript: *.ps. For a non-postscript capable printer, say that uses PCL5e, the *.ps file has to be filtered into PCL5e - so the printer will understand it. PCL5 (aka ljet4), PCL5e(xtended), PCL6 (aka pxlmono) are ghostscript based. If your router has the RAM, your router would need cups and ghostscript to act as a print server for raw (unfiltered job) from any computer on your network. In this case, you would select the "raw" print driver in your Windows setup. "Raw" does not filter, it just passes the job through.
Windows *.ps -raw-> router cups filters to PCL5e -> HL-5040 receives PCL5e
You can run into problems if you filter a print job twice; once at the windows machine and again at the routers print sever.
Alternatively you can filter *.ps to PCL5e on the computer and have the router print server pass it "raw" to the printer. If you are using a Windows "printer driver" to filter, then select "raw" in the cups interface on the router.
*.ps -> Windows print filter to PCL5e -> router print server -raw- -> HL-5040 receives PCL5e
A last option is to look into Samba sharing a printer.