Picture of PCB:
TTL serial port are probably the four horizontal holes on the left.
Unit only have two status LEDs, power and traffic/activity.
The router case is easy to open, once you've removed the two screws (on the bottom of the unit), simply pull it apart. There are however two or three molded latches on the inside (front). They will break (and void your warranty for sure), if you open the router.
Trendnet have made their own release of the OpenWRT FW on the routers support page, it includes file server (samba), printer server and some other features. You can install the latest OpenWRT beta over Trendnets version, but a reset seems to be required.
The beta FW of OpenWRT requires you to manually install the kmod-ath10k, ath10k-firmware-qca988x and kmod-ath9k and kmod-ath9k-common for WiFi to work (9k is for 11n, 10k is for 11ac). You'll need a wired connection (or an USB flash drive) to be able to install the required modules.
The ath10k firmware shipped by OpenWRT doesn't work very well. I'd suggest you use the firmware-?.bin (and perhaps also board.bin, not sure) from the firmware created for/by TrendNet. It can be DLed from http://downloads.trendnet.com/tew-823dru/firmware/openwrtfw_tew-823dru_v1(bb1_23_2-r76177).zip, or http://www.trendnet.com/asp/download_manager/inc_downloading.asp?button=Continue+with+Download&Continue=yes&iFile=27907 . Unzip the zip, then extract one of the bin files by using binwalk then unsquasfs on the .squashfs files (ref https://habrahabr.ru/post/264843/ ), until you end up with an extracted file system. Use the two .bin files from the extracted /lib/firmware/ath10k/QCA988X/hw2.0 and put them in the same dir on the router. You need to rename the current firmware-?.bin to something different, like firmware-?.bin.old. Same goes for the board.bin.
The firmware v10.2.4.48 from https://github.com/kvalo/ath10k-firmware/blob/master/QCA988X/10.2.4/firmware-4.bin_10.2.4.48 works very well as well, don't forget to rename the file to firmware-5.bin after DLing.
Remember OpenWRT now defaults ssh instead of telnet when logging in after a fresh install.