if we have change to our own router, then they unable check that already right?

I recommend you make a voice call via100 and choose the fourth option (Products and promotions), from there it connects directly to the sales department, rather than to customer service. then, they will also ask for your unifi ID to confirm whether you are eligible or not for this exclusive promotion, they will also tell you in detail about this promotion including the terms and conditions

they will recommend you three exclusive packages

300mbps Loyalty Super Value - RM 159 (FREE MESH M4 + STD20)
500mbps Loyalty Super Value - RM 199 (FREE MESH M9 + STD20)
800mbps Loyalty Super Value - RM 239 (FREE MESH M9 + STD20)

Just do some network utilisation observation.. hourly, weekly, etc to get the usage trends.

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Example - my daily utilisation. 100mbps is more than enough. 

As for upgrading / downgrading, it's possible, but be prepare to be locked into 24 months contract for every plan changes. 

Both DoT and normal is the same for me. Only difference is DoT higher initial connection latency due to TLS connection setup to DNS server. From my tests, averagely, Cloudflare DNS is now 2x slower than Google DNS in returning a reply. Cloudflare also returns Singapore IP for some CDNs instead of KL IP because it doesn't support ECS, causing video streaming to buffer alot because right now there is an issue connecting from Unifi to Singapore CDN server which goes all the way to HK and back according to traceroute by previous poster.