I have a 100Mbps connection but
I can never reach that kind of speed by wifi 2.4ghz.
max I can get is around 20-30Mbps only.

I can reach 100Mbps using lan and 5ghz
but many of my devices not compatible.

If you all can get that speed by wifi 2.4ghz
please guide me. Thank you

Your answer right there.
no reason full speed with other router but asus cannot.

If you ask asus, they will tell you it is 2.4ghz interference.
wrong setting, ask you change channel, move position, disable ipv6, tok kok sing song.
end up rma take few months
same thing after that, ask you change more expensive model.
boycott asus long ago edi

you go asus thread there.
read the last 100 pages,
then you come talk.