to alter via the advanced settings

find you way to (I used WINSCP to find it)

/usr/lib/lua/luci/model/cbi/cgminer/cgminer.lua

that has the frequencies that are available via the user interface


conf:tab("advanced", translate("Advanced Settings"))
pb = conf:taboption("advanced", ListValue, "freq", translate("Frequency"))
pb.default = "18:218.75:1106"
pb:value("16:250:0982", translate("250M"))
pb:value("17:243.75:1306", translate("243.75M"))
pb:value("17:237.5:1286", translate("237.5M"))
pb:value("17:231.25:1206", translate("231.25M"))
pb:value("18:225:0882", translate("225M (S3+ default)"))
pb:value("18:218.75:1106", translate("218.75M (S3 default)"))
pb:value("18:212.5:1086", translate("212.5M"))
pb:value("19:206.25:1006", translate("206.25M"))
pb:value("20:200:0782", translate("200M"))
pb:value("20:196:1f07", translate("196M"))
pb:value("20:193:0f03", translate("193M"))
pb:value("23:175:0d83", translate("175M"))
pb:value("27:150:0b83", translate("150M"))
pb:value("33:125:0983", translate("125M"))
pb:value("40:100:0783", translate("100M"))


are the ones available by default on the latest and greatest firmware