I have encountered a problem when trying to flash this router with DD-WRT. In AsusWRT, it says "To comply with regulatory amendments, we have modified our certification rule to ensure better firmware quality. This version is not compatible with all previously released ASUS firmware and uncertified third party firmware. Please check our official websites for the certified firmware."
Thanks, folks, for your replies. I successfully flashed my RT-AC88U with build 47528 of DD-WRT by using rescue mode and ASUS Firmware Restoration. It works beautifully and gives me a huge boost in speed over my old RT-N66W. I really appreciate the help. Without it, I never would have figured this out. MANY thanks!