Help! Alliance Broadband Router Issue – Adopt Nettech

Hi everyone , I need help regarding a weird issue with Alliance Broadband.

Recently took welcome 100 plan and they provided a dual band router. I tweaked the dns for control D adblocking.

Router – Adopt Nettech AD2X 2021

Though the 2.4Ghz works flawlessly but the 5Ghz wifi gets very slow ( like 10-15 kbps ) for a few seconds/minutes when it goes into idle state. Like my phone stays idle for 15 minutes , for first couple of minutes the speed is pathetic.

Reconnecting after disconnecting to the same network solves the problem.

So I am thinking, may be I got a poor router or defective router.

Points that may contribute –

1.There is a concrete wall between the phone and router. 2. DNS changing didn't cause it, because 2.4 Ghz one works fine. 3. I can't afford upgrading router now ( effective cost for router was 350, new ones almost 2k )

Please help to sort this issue.