Samsung S20 WiFi problem

se2020

I have a Samsung S20+5G phone.

One thing that might be worth mentioning/relevant to this problem is that I got this phone cheap from a pawn shop. The previous owner had not paid the bill and the sim1 slot is blocked/locked. 
I can make calls and use mobile data ilwith my sim in slot 2.

The issue, the phone will connect to my home WiFi but next to the wifi strength symbol is an exclamation mark. The wifi shows as 'connected - no internet'

I have tried various suggestions I found online like checking the date/time, mac & vpn settings etc. with no luck.
I've factory reset and installed latest updates. 

I can activate a hotspot on an S10 and the S20 will connect to that and work fine.

My broadband router is an ee brightbox,  it must be 10+ yrs old.
Is it likely the newer phone is not compatible with the old router?
I have connected a smart tv,  Samsung a20, fairly recent iPhone and so on to the brightbox with no problems so I'm a bit stumped by this!

mta999

Do you have another router you can try for example a family member or friend or even Starbucks etc

If it was me I would be trying to connect to a selection of routers to see if the problem occurs with other routers or if it is just my home router that causes the problem

Cisco001

Test the S20 wifi somewhere else to confirm if that is hardware problem of phone or router.

What is the model of your existing router? May be it is time to upgrade...