That's abysmal! Every 5GHz Wifi AP I've ever come across lets you run both PHYs at the same time.
Please, on my behalf, sternly talk down to your router.
Even ignoring the massive issue of device compatibility: 5GHz and its protocols do not have anywhere the range and penetrating power of 2.4GHz. When I walk outside my house I can keep watching videos, but my laptop does this by transitioning to the 2GHz radio link modes.
Whenever you replace it, get a separate AP and modem.
Also, DOCSIS 3.1 modems come with queue management. You may benefit significantly from an upgrade even if nothing else changes.
> only 5ghz is performant
Most of these barely need any bandwidth. A printer is possibly on the higher end of bandwidth, but I think the number of printers that support 5GHZ is possibly still single digits.