I realized that while doing the setup i set the dns to google dns at internet part,which an IT friend told me for that part better to use the ISP dns. Now i set it to ISP dns, and set the google dns at the dhcp server part. Still monitoring now,but my 5ghz speedtest looking normal now.

scan the airwaves and move your channel; maybe you have a bad clash with neighbour over same channel.

adjust width of channel to be wider; but avoid using 5GHz width 160MHz to be considerate to neighbours.
ie. avoid use so wide until other people cannot even get a quieter channel to slot in.

I'm currently using AX73 @ Unifi 500Mbps as well, here are my settings

transmit power: high
channel width: 20/40/80 MHz
mode: 802.11a/n/ac/ax mixed

2x2 and above devices no problem getting the maximum speed.

the situation is fluid.

you setup auto channel this, auto setting there.
your left neighbour do the same.
your right neighbour do the same.
you front neighbout use Asus.
your rear neighbour use Deco.
some fucker simply allow auto 160MHz.
another fucker set static channel and 160MHz.
your mesh nodes also use different channels for each of them, only the backbone will be same channel.
all your auto devices also will auto-reselect a new channel and width from time to time according to their own algorithm on doing so.

it cannot be helped that from time to time, you must experience hardship when too many APs ended up fighting for same channel.

Perhaps it's congestion like what Oltromen said. Neighbours also change package, meddled with the settings > 160MHz.

Before concluding, check your LAN cable from ONU to Router is properly fitted in their respective slots/ports.

Story: Yesterday got technician come to check my complaint about calls not stable/clear, higher ping to TM servers, Unifi TV laggy. The bugger wiggle rigorously on the LAN cables that are connected to the router, after that he held on the LAN cable that's connected to the WAN port, he kept pulling in/out slightly saying you see the indicator light, that means no internet. So the guy's hinting my router is the problem. I told him I've tested with another router and cables, the problem remains, that shut him up. At that point, I went up to my router and check myself, lo and behold he has loosen all my LAN cables from their respective ports. I just told him off, and change a new ONU. ONU changed, calls loud and clear, lower ping to TM servers, unifi tv no more laggy.