After my initial disappointing experience with the wAP LTE kit, I saved up for the Chateau LTE12, and as far as performance goes, it exceeded my expectations - about 1.5x faster than my previous Huawei B535, so I'm keeping this one
I've checked my mobile signal strength stats on the router and from what I understand, there's plenty of room for improvement. My signal strength metrics on average:

• RSRQ: -7 dB
• RSRP: -112 dBm
• RSSI: -84 dBm
• SINR: 12 dB

The only station I'm using is approximately 400 meters away, but between it and my router (which sits in the corner of my room) there are 2 concrete walls and a handful of trees. Is there any hope for improving signal strength with an indoor antenna mounted somewhere close to the router, or is this not worth even attempting and I shouldn't even approach this topic without being ready to mount an antenna outdoors, possibly on a roof/mast?

Is there any hope for improving signal strength with an indoor antenna mounted somewhere close to the router, or is this not worth even attempting and I shouldn't even approach this topic without being ready to mount an antenna outdoors, possibly on a roof/mast?

Depends on geometry of the obstructions between base station and your Chateau, use of indoor antennae might or might not help. Placing antennae so that there's line of sight between them and base station is the only sure way of improving things. Beware that antenna cables usually used for client devices (a variant RG-50 cable) comes with some attenuation and using cable between antenna and modem with length over 10 metres (30 ft) is discouraged.

Yeah, I'm aware it would be best to have a clear line of sight, but unfortunately that would mean getting like a 15-20 meter mast up just to get past the trees. I'm not sure I want to go this route.
If I were to try some sort of antenna, would you recommend any? The Mikrotik docs mention the mANT LTE 5o, which seems to be a omnidirectional external antenna. Should I just pick an internal antenna with the highest gain I could find, or is there more to choosing one to work in my case?

Small update: I have bought an antenna - Signaflex Dual X-Cross (2x22 dBi), which has been recommended for me by a distributor on a local auction site. They had good ratings, so I decided to give them a shot. PS: They recommended this antenna instead of a directional one because according to them, "a directional one wouldn't have made much of a difference at this distance to the station".
I plugged it into the router and put the antenna somewhere near it just to test, and unfortunately, the change in signal strength and quality has made me triple check if the antenna was even being used. It was. RSRP maybe improved by 2 dBm, but RSRQ and SINR didn't seem to budge.
Then I stuck the antenna out the window at arm length just to see what happens. This is when I saw the difference in signal, RSRP went from about -112 to -100 (which at least according to the Mikrotik Android app, isn't even enough to lift the RSRP line on the graph off the ground ), RSRQ from -11 to I believe -6, and SINR from 8-12 to 20. I did a single speedtest for comparison, and while the speeds themselves weren't really different (rounded, both 52 down/3 up), the jitter was noticably lower, especially for upload.
I'm considering mounting it on the rooftop (maybe like 8 meters higher than my room) on the weekend with no expectation of gaining speed, but with expectation of gaining some stability, since that's a little bit of a problem for me right now - dips in speed and uncalled for ping spikes.

With that kind of signal level I’d go for a directional antenna using the lower LTE bands. Be ware of the cable length though.

With that kind of signal level I’d go for a directional antenna using the lower LTE bands. Be ware of the cable length though.

In my case that would mean sticking to B20 and disregarding B3 and B1, right? Do you think it may be worth it to focus on a single frequency with a directional antenna instead of going 2/3CA with all of them using the antenna I just got?
I'm starting to regret being talked into not choosing a directional antenna by the distributor...

As for cable length, I'll need 10 meters at least, and even with that I'll probably need to move my router closer to the roof.

Settling on this antenna. Mounted on the roof, shooting through the trees to the station and talking to it in B1 and B3, the final signal metrics are:

• RSRQ: -12 to -9
• RSRP: -93 to -100
• RSSI: -69 to -61
• SINR: 10 to 24

Despite RSRP still being fairly low and all metrics flailing around, I'm seeing great gains. In terms of download speeds, before the antenna I was getting 24-32 Mbit/s on average, and the highest I've noted was about 64 Mb/s. Now I have 64-80 Mbit/s on average, with the highest I've noted (but also much more frequent, especially when downloading from Steam) being 124 Mbit/s. Uploads have also improved, even if they're still lower than I wish they were (from ~1-3 Mbit/s to ~4-8 Mbit/s). Pings are much more stable now.
I've also approached the station with my phone, with direct line of sight and about 15 meters away from it, my phone scored 140 Mbit/s on download and 25 Mbit/s on upload. So, I'm guessing as far as download speeds go, I'm getting great results, but somehow upload speeds are still lagging behind. But I'm content

Thanks everyone for your input. Even if I didn't go with a directional antenna as many of you have suggested - with the manufacturer giving an opposite suggestion I was getting perplexed in what to choose. If I didn't spot enough of a difference with this omni antenna, I would have most likely returned it and got a directional one per your advice, but I'm content with these improvements.