See 1.2gbps how? Like connection settings or actual wireless traffic?
That said, keep in mind that wireless is strongly misleading. There are several factors that determine your connection speed.
Number of antenna chains
Since there was no information, I'll try to take a guess in the dark about the issue. I'm going to assume you have the Asus RT-AX88U. This device supports 4x4 antenna chains, 160mhz wide channels, and up to 1024QAM.
So let's look up the handy chart that's on wikipedia.
You'll see MCS indexes on the left hand side. These correspond to increases in speed directly proportional to signal quality. As your signal level goes up, the devices will automatically move up and down each one of these levels. You will only get to MCS11 when you have like 90% or better signal (Max bars on whatever is displaying them). As you move away from the AP, the devices have to slow down the speed at which they work in order to keep the connection going. So at say "1 bar" you might be using MCS 1, which at absolute max speed is 144mbps. So even though you have a shiny new Wifi 6 device, it might not actually be faster than an AC or possibly even an N device. AC and N share the exact same MCS table, but AC has MCS 8 and MCS 9. AX has slightly better efficiency, so it's slightly better at the same indexes. It also has provisions for MCS 10, and MCS 11 using 1024 QAM. The thing is, 1024QAM is going to require a VERY good signal to be able to take advantage of it. It's very possible you'll only see that with no obstructions between the AP and the client. So while both devices might be capable of it, keep in mind that range plays a huge factor in speed. Anything that was designed for an enterprise environment knows that less range is better because it allows denser deployments with higher signal strength. You simply don't want to have your AX connection going through 4 walls otherwise you might not be able to take advantage of the increased speeds it provides.
So that said, the chart of AX doesn't list number of antenna chains like the older charts do. So that chart is simply for 1 chain (spatial stream is the proper term). If your client device is 2 x 2, then you just need to take whatever is on the chart and double it. If you client was 4 x 4, then you could quadruple it. But also if your client was 4 x 4 and the AP was only 2 x 2, then you're only getting 2 chains.
So a real world example to get 1.2gbps connection rate would be something like this:
MCS11, 80mhz wide channel, 800ns guard interval. That would yield 600.5mbps, but then you need to double that, so you would have exactly 1.2gbps connection rate. This is most likely what's going on. I would expect the default configuration to be using 80mhz wide channels and not 160mhz wide channels. The reason being that AC doesn't support 160mhz channels on most devices. So to maintain backwards compatibility, the device likely uses 80mhz by default. This would mean you're getting half the bandwidth. But that said if you need to support AC devices, you simply might not be able to turn on 160mhz wide channels, or it's very possible that your AC devices can no longer connect to 5ghz to the AP. And definitely N devices aren't going to work correctly at all in 5ghz, so they will have to operate on the 2.4ghz band.
Another point is that the connection rate is 1.2gbps, but actual traffic flow used to be about 60% of that for anything using WPA2. It will be interesting to see if the total throughput goes up with WPA3. But generally that just means a 1.2gbps is good for about ~720mbps of data throughput in one direction. (Wireless is still half duplex meaning it and only upload or download and can't actually do both at the same time. It has to stop downloading to upload and vise versa)
Side note: I use the term chain and not just antennas because originally Wireless N devices had 3 antennas on them, but it only supported 2 separate connections. The 3rd one was for diversity to improve signal strength. Likewise G used two antennas but only supported a signal chain / spatial stream.