Hi @DrubUK
Welcome to the Sonos Community!
The fact that Alexa works and Spotify plays very strongly suggests there is no problem with the Roam, or its ability to remain connected to the network. It is not experiencing any issues.
The problem lies with the communication between the apps (Sonos & Spotify) and the speakers - this is either due to router configuration, a router fault, network topology, or firewall/VPN software on the phone.
As you don’t have any extenders, the topology is likely simple so I recommend you switch off your router for at least 30 seconds, and repeat this several times a year whether you encounter issues or not. It may also help to reserve IP addresses for your Sonos devices in the router settings to prevent other devices from accidentally getting the same IP address as one of them, which could definitely mess things up in this way.
With routers, you get what you pay for, and forgive me if I am wrong but you did not pay for your Smart Hub 2 at all - you pay for the internet connection. As a result, most home routers in the UK - where this is common practice - are below par due to the costs of handing out literally millions of devices for what is basically free - they obviously cover their costs, but will still try to keep costs down. An option is simply to replace it - I would not recommend paying less than about £$€50.
If a reboot of the router does not help, please check your phone’s firewall/VPN software, if applicable.
Please do not reset your Roam as suggested above! It will not help.
I hope this helps.