So i just installed 300mbps fiber package yesterday.
Modem and router is at dead center of the house downstairs with no things blocking it.

Sitting 3-5m from the router also nothing blocking, i get full speed 300mbps on 5ghz.
BUT, same distance when connecting thru 2.4ghz i can only get 50-60mbps!
The tech that came and install said its normal and to be expected.
WTf !

I sure as hell do not believe this!

When i had TM 100mbps before....both channel same distance i could get full speed.

What is wrong here?
Some ports/function/speed blocked by Maxis that force us to use 5ghz band?

But as you all know 5ghz cannot travel far, so when i go upstairs i either cannot connect or forced to use 2.4ghz which is slow.

Which router they gave u

TS, it may be due to the router. ISP are known to give out mediocre router. I would like to suggest you have a look at the router discussion topic at:
https://forum.lowyat.net/topic/4658096

They just gave the the standard router that Maxis supplies, some black color one which stands up.

Yup down along the line am thinking of upgrading to a MESH network system and have another point upstairs.

Thanks for the router discussion link, will go and check it out

Unless you literally live in a jungle with no surrounding interference, you aren't likely to achieve close to 200 Mbps on 2.4GHz.

2.4 GHz is known not to give full speed in any other way.
Want full speed, 5GHz it is.
In other words, 2.4 GHz: Coverage > Speed
5 GHz: Coverage < Speed

But previously with Unifi i was at least able to get 100mbps on an OLD router that TM provided.

But now with a newer supposedly improved router i am getting 50mbps on a 300m package?

Kantoi wei hahahaha

Your 2.4GHz wireless connection with 20MHz channel bandwidth is restricting your speed.
Technically, it gets you only upto 75Mbps but there's overheads which will lower it significantly to around 50Mbps .

There's no way you're going to reach anything near 300Mbps with 2.4GHz wifi without investing in better hardware which uses 5GHz or at least 2x2 MIMO 2.4GHz at both ends of your router and wifi adapter/receiver.

I have already changed the bandwidth to 40mhz.
Max i achieved was 56mbps.

Down the line gonna upgrade to a MESH system.

But for the 5ghz bandwidth, leave it at auto or certain freq?

If despite using wider channels for your 2.4GHz wifi and you're still not connecting close to 100Mbps(max speed after overheads), yet still connecting similarly to 20MHz bands, then your wifi devices might be to be blamed. Either they're too old, incompatible with your router's chipset or do not support wideband 40MHz 2.4GHz standards usually those China unbranded OEM devices suffer from such problems because they don't follow proper standards.

Remember your receiving wireless clients/devices must also match whatever your router throws out. No point buying the best routers out in the market when you use those cheap China OEM tablets to connect.