Without more information about your router/network, the easiest thing to do would be to configure the SSID and password of the new router the same as the old router. Devices will normally reconnect automatically.

But if you have done a factory reset on the One, you will have to set it up as a new speaker. I would suggest you find an ethernet cable and hardwire the one to your router to get it setup if you still are unable to connect wirelessly. 

Make sure you have the latest updates once it is setup.

Thanks for you reply. I’m not very technical when it comes to router settings etc. 

So basically I change internet provider and upgraded to fibre broadband. 
Most devices such as Amazon echo devices are now on the network and work fine. I just can’t seem to get the Sonos One to connect only with Ethernet cable.

In the app, go to settings > Network > Manage Networks and select Update Network. You should be able to remove the cable after that

​@Pools-3015 , I am also having this same issue and have tried the steps you suggested but says it can’t find any nearby devices

I did a factory reset on the speakers and added them to the network.

Since may 2024 a factory reset is required to ad a new network on older Sonos devices, among which is the One (Gen 1). See

Connect Sonos to a new router or WiFi network | Sonos

If there are no safety concerns that would prompt you to need a new SSID/password, you could have ste up your new router with your old SSID/password. The Sonos system in all probability wouldn't have noticed the router change then.