Hello, I need a recommendation for an LTE antenna for the Chateau LTE6 AX router. The router is currently inside a metal rack, and we require a small LTE antenna that can be mounted on the rack. Additionally, what type of SMA cable is best to connect the mANT LTE 50 antenna to the Chateau LTE6 with a 5-meter length?"

... we require a smart LTE antenna ...

What is your definition of word "smart" in this context? In UK smart means "having a clean, tidy, and stylish appearance" while in US smart means "intelligent, or able to think quickly or intelligently in difficult situations" ... I'm affraid that Chateau LTE6 doesn't support smart antennas according to US meaning[*] but you may be able to find some posh antenna if you're after UK meaning.

If you're talking about "high-gain" antenna for LTE part of your installation, then yes, it's possible. Antennae are passive elements, so you can connect any 3rd party antenna (you need a pair of strategically placed antennae or a 2x2 MIMO antenna to make most out of it), the only unknown is antenna connector type on Chateau LTE kit ... so hopefully someone who knows this info will chime in or you can directly ask Mikrotik (by sending them an e-mail to support@mikrotik.com; this forum is not official support channel of MT).

[*] Generally smart antennae are those that actively track link peers, which mostly means antennae with proper beam forming capability. In mikrotik world those are products from 60GHz department. But this also means that antenna is (semi)permanently attached to radio (if radio is not integrated in the antenna) because each antenna element has to be directly driven by corresponding radio. And a pair of coaxial connectors/cables simply don't support such connectivity.

... we require a smart LTE antenna ...

What is your definition of word "smart" in this context? In UK smart means "having a clean, tidy, and stylish appearance" while in US smart means "intelligent, or able to think quickly or intelligently in .

We need small not smart*(autocorrect change)

Beware of small antennae, usually antenna gain is inversely proportional to antenna size.

An idea: since your problem is that device itself is inside metallic housing, why don't you re-use original antennae. only use cables of appropriate length? Depending on cable quality, additional loss is around 2-5dB per 10m. So if LTE signal strength outside the metallic housing is decent (at least -85dBm), you could survive with a few dB of additional signal attenuation. If LTE signal strength is lower than that ... then you need antennae with decent gain and that rules out small antennae. (But, again, it depends on your definition of "small" in this context)

Is the plan to run the cable outside, or just to get out of the metal rack?

Some notes:
- There are standard SMA on back of Chateau LTE6
- 50 Ohm cable is what you need... @mkz talks about length... since each meter/foot added increases loss (less signal). It being 50 ohms is what's important. Any cable you buy should have some table of loss-by-length and if not, don't buy it. And, obviously lower loss cables are thicker and more expense, but it loss never goes to 0.
- Antennas like free space. So if indoor, getting to top of rack closer to ceiling or toward a window may be enough. If outdoor, knowing where the towers are become important.
- Not sure antenna maker brand matter that much... BUT if someone doesn't have polar plots... I wouldn't buy the antenna — my thinking is if you can't bother publish plots, maker may not have done good testing. I'll say I've used Taoglas and Poyning antennas and had good luck with them (but I'd put them in the "posh" category). But even Mikrotik $20 mANT-5o OUTSIDE the rack may offer a lot benefit.