Have a look at this (rather old) tutorial, an interesting solution is presented starting at 15:15. He’s using an additional MY_ROUTER - ISP_ROUTER ethernet connection to bring the lan subnet to wireless devices connected to ISP_ROUTER. Only works when there’s no DHCP on ISP_ROUTER so that connected devices get their IP from the lan. I've had this running without problems in the past, before rearranging my Wifi setup not needing the ISP router’s Wifi any more.
