Hi,
Is it possible to use external antenna with pigtail https://mikrotik.com/product/acsmaufl for Hap ax lite LTE6 (L41G-2axD&FG621-EA) https://mikrotik.com/product/hap_ax_lite_lte6?
As designed: no.
See this thread where someone made pictures of internals for that device.
The device is designed to be used indoors where signal is good.
If you are in a bad signal zone, you should use our SXT LTE or wAP LTE products
Looks like the red wires are used to connect LTE antennas. They’re not present on regular hAP ax lite from what I’ve seen online. Now I’m tempted to buy this and try to replace them with the pigtails. If this works then it would make this device near perfect for a certain very specific use case that I have. The only problem would be the inconvenient shape of the vertically standing enclosure. Something like what was used for the hAP ac2 would be much better for me, in fact I think that solution is genius and should be used for all home gateway routers.
Roughly 100 EUR or about as investment 
There is no modem on the “regular” hAPaxLite, so no antenna connectors.
FWIW, If you did get a hAPaxLite-LTE6. You’d be drilling custom holes to get the SMA bulkheads out of the unit (and likely voiding warranty). But Mikrotik sells the pigtails that go from the U.FL on the board to an SMA connector for antenna: https://mikrotik.com/product/acsmaufl
There is wAPac-LTE6 as an alternative, that can mount on a wall. Still need pigtails to get an external antenna, but it has pre-drilled holes for it.
I like doing this kind of modifications and probably voided a lot of warranties already.
Still not quite for my use case. The absolute best would be something with all the features and the size of hAP ac2, but with an LTE modem and external LTE antenna connectors, preferably also internal LTE antennas for where the signal is good enough that I don’t need external ones, so that I can disconnect them when I don’t want them to get in the way but still have the option of connecting them when I need to have better connection. The hAP ac2 with an external USB LTE modem would also be acceptable, but it’s hard to find any that don’t have that HiLink crap and would actually work with the router and give it a public IP.
Actually the hAP ax lite LTE6 doesn’t look as good now that I look at it, as it doesn’t have 5GHz Wi-Fi. I also discovered something called RBM33G that would probably be better for my needs, though I would have to make my own case for it and I’m not sure what size the final result would be.
This is probably getting too off topic for this thread.
There is a new board
L11ug or something like that.
http://forum.mikrotik.com/t/new-l11ug-5haxd/171943/1