Already done.

Flashed the time modem to eu firmware and use inspect element to evoke the hidden wds page. Linked the router with my main rputer via 2.4ghz wds. Somehow this c1200 cannot detect my 5ghz main router set using 802.11ac mode only.

Everything working fine now. Can get dhcp from main router. Just cannot wds using 5ghz unless i set my main to 802.11b/g/n/ac mode. Which i dont want cox i wan the main router to run full AC at 5ghz.

Yup. I actually had a same router preciously but that one always hang
. Every 1 week had to restart router. It seems like this TINE one doing ok. Dunno if they revised the router.

I dunno. Fail for me cox i set main router to use AC mode only. However that main router 5ghz is on channel 112. Whereas the secondary router which is the c1200 is flashed to eu v1, and it support a few channel only. Maybe thats why the c1200 cannot detect 5ghz for wds.

Due to fed-up with AC1200 few month back, I bought the tp-link 2300 and it running stable and solidly. Give up for AC1200 and keep in shelf now. lol.

ac2300 far more better than ac1200, and the price are big different too. 
The MIMO really make the 2.5g and 5g wifi good experience vs 1200, which is really suck.
The 2300 with far better cpu and ram, and better hardware. It did a good job, and designed for multi-users high demand home usage (not for gamers, as gamers expect far better throughput). There is build-in Trend Micro internet security feature, even with that running, the cpu usage is like 2-10% majority of time.

The 3150 is fairly old model and going to replace soon, I prefer 2300 as it using 1.8GHz dual-cores in-order 64-bit ARM Cortex A53 cpu, same per ASUS RT-AC86U, but cheaper price due to lower wifi bandwidth and etc. I feel this is good enough for my family with 4-6 members on 100mbps unifi.
CPU1: Broadcom BCM4906 (1.8 GHz, 2 cores)
FLA1: 128 MiB (ESMT F59L1G81MA-25TG2Y)
RAM1: 512 MiB (Samsung K4B4G1646E-BYK0)

Read my comment above to bro ikengkai.

The 5G speed are better, but the coverage not as good per the 2.5G. In far end of my house with 3-4 walls, the 2.5G still good signal, but 5G pretty bad and sometime drop. I think even the 2.5G good for normal smartphone usage, but for laptop access, the 5G will be better if heavy file transfer/streaming.

Page 29 for WDS.

Yes, can flash back. Page 1.

Don't.
Stick with Channel 36.

The lower, the better.

Yes.

Flash to Edit7 firmware for AP mode. It works well. Connect by lan to an Asus RT86U as base router.

Really can see the difference between Asus and Tplink

Not planning to use WDS coz I wired my house with Cat6 cable every room. Also using WDS what I read is that it takes up additional WiFi bandwidth vs AP connected by LAN

Is TP-Link C1200 better or the new TP-Link EC230-G1?