Unifi Air will be the same or very similar like the usual 4G LTE service you used by usual telcos like Maxis, Celcom, Digi, U Mobile....
The coverage is expanding, but not everywhere yet, and it definitely takes time.

If Unifi Air is not available in your area yet, I would advise to give up considering about that, and go for other telcos (e.g. Digi, Yes 4G LTE, U Mobile) that has better 4G service in your area and subscribe to their wireless broadband plans, whichever suits your data usage and budget.

Or stick to Streamyx (Unifi Lite) for the speed up to 8Mbps, and RM69/month. There's isn't much you could do.

Is the Unifi Air's SIM card "phone number" begins with the prefix "015-xxxxxxx"?

As what I know, 015 prefix are only for wireless broadband use, and can't be used for making and receiving regular mobile calls. It applies to all telcos.

Hmm, if it's 010-xxxxxxx, there is a chance that TM could activate voice service if they want.

You have to report this problem to TM so that they can solve the issue for you.

You check on the supported 4G frequency bands for both huawei modems, on the packaging box or user manual, see if it supports any of the bands used by TM Unifi mobile below:

LTE FDD Band 5 (850MHz)
LTE TDD Band 38 (2600MHz) or 40 (2300MHz)

Then you know which frequency bands that unifi mobile uses in your area, as well as knowing your huawei modems supports at which frequency bands.

Unifi Mobile = Unifi Air

True also.
Don't forget, 65d also supports b28 (700MHz). TM might interested in getting some of this spectrum after analogue switch off has completed and spectrum auction begins. If that really happens, Unifi air users that using Huawei b618-65d modem can get to enjoy service from this 700MHz band right after its base station is up and running, on top with 850MHz and others.

Hmm... This is something that you may not able to contol.
Same like mobile telcos, speeds are on best efforts basis. It depends on the location and the network traffic in your area.

Unifi Air and Unifi Mobile are the same thing, and coverage is also the same. It's just that both services are cater for different user needs. Marketing.

By right, no.
But they can cap the speed if they feel like it.
Since they "promised" to get an average 20Mbps, then just accept as it is. If your speed is lower than 20mbps, then you may ask TM what's going on.

If "Check my coverage" tells you there's no Unifi Air coverage at your location, then it is safe to say that you are advised not to consider subscribe Unifi Air for the time being then.

Good Celcom 4G coverage does not necessarily mean Unifi Mobile will roam Celcom's 4G network whenever Unifi's own coverage isn't available in the area. Also, whenever your Unifi Mobile shows 3G or H+, it is roaming on Celcom's network.

You've already got your own answer...

What is your phone model? Check if your phone model supports these 4G LTE bands, which are Band 5 850MHz, Band 38 2600MHz and Band 40 2300MHz. If all these 3 bands are supported, yet you get poor network performance from Unifi Mobile at your area, then it is advised that you give up and switch to other telco for the time being.

When you force 4G/3G that way, do double check that you're still able to make/receive phone calls.
1-2 signal bars is consider quite poor already, and that could be the reason your phone fall back to 3G. Yet, you mentioned that it'll disappear as well, which somehow shows that the network is unstable in your area.

Your phone supports 2300MHz Band 40 and 850MHz Band 5, but doesn't support Band 38 2600MHz.
Well, if so happen Unifi Mobile's coverage is good in your area, but only on Band 38 2600MHz frequency band, and not so well perform on other bands, then too bad for you lo... Try look for your family or friend's phone models that supports all 3 bands, then do all the network tests, like monitor signal bar/readings, speed tests, video streaming and web browsing, etc. Then at least you'll know the root cause of poor network performance comes from where.

My advise, give up and move on to other telco.