I suppose the applicable model is actually WA855RE

From the listing

https://www.amazon.com/dp/B0195Y0A42/ref=cm_sw_em_r_mt_dp_EF03MD1SZFKK5E38M44V?_encoding=UTF8&psc=1

>Wi-Fi Coverage up to 800 Square feet

depending on OP's space, that may be on the too small side.

One option is to buy an 2.4GHz-only extender like this. It's $20 at Amazon.

You set the smartbulb to connect to the extender - which is only able to use 2.4GHz - but the extender is connected to the main network and is smart enough to connect to the mixed 2.4Ghz\5Ghz main network.

I don't know as an absolute certainty that it will work with your Spectrum router, but it's worked perfectly with my Linksys Velop and Netgear Orbi mesh systems, which also have separate 2.4\5Ghz networks but use the same SSID.

Incidentally, the reason the mesh networks (and, I assume, Spectrum's router) do this is because most client devices can automatically switch to the best network as needed. For example, my phone uses the 5GHz network when I'm inside the house, because 5GHz is much faster... but it automatically switches to 2.4GHz if I go outside, because the 5GHz doesn't have the range for that.

EDIT: many routers can also act as extenders\repeaters, so if you have an old 2.4GHz router lying around, it might be worth it to look into that. The only reason I even had the TP-Link extender was because of a dead spot in my old house with an otherwise excellent (for the time) Linksys router.