I will start by my setup.
I am currently using the ASUS DSL-N14U to connect my DSL WAN into it
Another device SMC switcher is connected to the DSL-N14U with very very long 50 meters RJ45 cable then another device (DSL-N12E_C1) is connected and placed near to that switch to work as a router that gives another wifi network in another erea. (The setup won't work without the switcher between 2 other devices because of the long RJ45 cable however it worked when I used DSL-2740U intead of the DSL-N14U as my main DSL modem, but I prefer the DSL-N14U so I kept it with the switcher)
All of that is for just 2MBit internet speed and only 512 KBit upload... I can't get better speed.
Now I want to manage the traffic properly on that low speed bandwidth so I tried the QOS feature in my ASUS DSL-N14U to prevent bufferbloat for online gaming and set aggressive rules to highly prioritize certain ports that my games use and lowered everything else, but as soon as someone open his internet browser I start to lag heavily even if it just uses a 10 KBits of the bandwidth so I tried to lower my bandwidth but that does not seem to work so I am going to buy another DSL device that is supported by OpenWRT because I have been told that OpenWRT is capable of managing the traffic without even losing any bandwidth, but I still has a question: if I set rules in SQM of openwrt firmware, will that apply on all devices connected to main WAN DSL modem/router (through the switch or through the other wifi network or whatever) I am also not sure if any system can manage my low speed network as my games uses almost half of the bandwidth so will that be a problem?
I don't understand why. If I remember well, a Cat 5e cable should be fine for up to 100m.
I think you have enough ADSL devices already. If you want an OpenWrt device then yes you can get a supported ADSL modem-router, but there is a relatively small number of these, due to drive unavailability. You might keep the best ADSL modem you have, set it in a bridge mode, get an OpenWrt router and set its WAN interface up as PPPoE.
My gueses were right. I can't manage anything in my low speed service. Thx for info. I think I will have to to assign for another service on another line and use one for gaming and the other for other purposes since I can't get better speeds.
SQM makes an amazing difference on DSL. It often is sufficient to set it on the WAN interface just before the modem since that is the bottleneck point. It's also possible to throttle other users and let your gaming go unhindered to the WAN SQM.
Ask your DSL provider if they offer bonded lines-- this brings two DSL lines to one modem for twice the speed and it may be less than twice the cost. The provider's modem must be used there.