@bvhalm 

Im guessing the Deco is connected to your ISP router?  

If so it could be NAT that is causing this and you may need to forward ports to resolve.   

Leave the Deco in ROUTER mode unless you have cables ran between the room, Access Point mode cant connect to eachother wirelessly, they are for cabled setup.

Stick it back in router mode and make sure it working, then check the ports required for your game (you should be able to find these mentioned online).

Say for example its ports  12345 and 12346 then you need to log into your ISP router and set it to forward ports   12345 and 12346 to the Deco,   then do the same on the Deco for your Playstation.

There is loads of articles available for this, just look for NAT or Port Forwarding.

  @Philbert 

First of all thanks for telling me to use router mode instead of access point. I just tried using port forwarding, but the game still won't work. I did the port forwarding for the deco first, but that didn't work so then I used the IP my PS4 is on. I'm not quite sure if I can access any control panel of the Deco?

  @bvhalm 

Did you get the list of ports that need forwarded from the internet?   This will be different for each game, also make sure if they are asking for UDP or TCP connections. 

Ok so taking FIFA23 as an example, the website says the following ports

FIFA 23 - Playstation 5

• TCP: 1935, 3478-3480, 3659, 10000-10099, 42127
• UDP: 3074, 3478-3479, 3659, 6000

So what you need to do is go onto your ISP router (192.168.1.1 for example) and create a port forwarding for each of those. You can use ranges for this in most cases.   When asked where to direct this port forward, it needs to be to the Deco router, this will be 192.168.68.1 by default and should be the only thing connected to your ISP router. 

Once that is done, go to the Deco App and open Advanced, NAT Forwarding / Port Forwarding and repeat the same ports.  It will ask for an internal IP, which will be your PS5 IP address.   Repeat for all the ports mentioned above

Ok so a quick why this happens.  Routers use ports to recieve data, email for example uses 993.   So email comes into your ISP router on 993, its forwarded to deco on 12309, which then forwards it to PS5 on 12434 or something random.   By setting up these Port Forwards, you are telling the ISP router ANYTHING on that port gets sent to Deco on the same port.  While also telling the Deco the same instruction, ANYTHING on that port goes to the PS5

You do need to have both routers (ISP and Deco) set with the correct forwards for this to work.  You cant go straight from ISP to Playstation for example 

In short   the ISP routers forwards to the Deco Router..  The Deco router forwards to the Playstation. 

Hopefully this is your issue and its something you can fix! 

There is another option.. but its more messy.. 

Depending on your model of ISP router, you can set this for MODEM MODE only.  This will remove NAT on the ISP router and basically bypass this forwarding for you as everything will do to Deco.  

But you would need to check your ISP Router can do this, also may require some config on the Deco WAN connection.  It may require a username/password to work with the MODEM MODE.. sometimes not, it depends on your ISP and hardware.

  @Philbert 
So I did exactly what you very well explained. I port forwarded the ports on both routers now, but it still doesn't work. To my suprise I was able to play like 2 games when I only port forwarded the Deco router, but then it started to disconnect me from the games again. After port forwarding my ISP router as well, it still didn't work.

I did come to the conclusion it is not with every game I try to play, since Rocket League works perfectly fine, even though I haven't even put open any ports for that game, just for FIFA.