Is this a good router for a home with many Wi-Fi devices?

No. Its Wi-Fi is 802.11n only, so phones, TVs and laptops will see much lower speeds than with a Wi-Fi 5 or Wi-Fi 6 router. It suits wired branch networking better than whole-home wireless.

Can it use a 4G SIM as its internet connection?

Yes, it supports 4G mobile networks, typically through a compatible USB cellular modem. That makes it useful where fixed broadband is unavailable or as a failover link.

Does it replace a DSL modem?

No. It has no built-in xDSL modem, so DSL service needs a separate modem in front of its WAN port.

Can I add access points or a mesh system?

It does not support mesh. You can attach wired access points to its LAN ports, but they will not form a mesh with its own Wi-Fi.

Expect the router unit, rack-mount brackets and a power supply or cord. Check the specific bundle before ordering, since accessory contents vary by seller.

Is it worth it over a consumer router?

Only if you need rack mounting, 4G WAN and managed branch routing. For home Wi-Fi speed and easy setup, a consumer Wi-Fi 6 router gives more for less effort.