Your router outputs a WiFi signal over two separate bands. The 5Ghz WiFi band which is faster and has less penetration through walls etc. and the 2.4Ghz band which is slower, but far more reaching/penetrating.
At the moment your Era 100 is using the 5Ghz band and your other products are using the 2.4Ghz band - the obvious thing to try is what I suggested earlier, which is to put the Era speaker onto the 2.4Ghz band. Sadly it seems you’re not confident to do that, via the router configuration pages and don’t wish to read the router user guide - so I can only suggest you get someone into to do that for you and see if it then works.
Only if not, it just perhaps becomes a case of nothing ventured, nothing gained.
Edit: I don’t think you need a new modem/router, but that’s a matter for you.