Hi, gratulations with new LTE18 and closer relationship with best operator in Latvia (LMT) :)
LMT (ISP) as an additional service is providing static IP addresses, IPv4 or IPv6. IPv4 is easy, but please somebody write down beginners guide how to setup static IPv6?
It's LTE internet. ISP is usually giving a range ::/64. Now how to setup correct pool? Does it needs new route definitions? How to configure router IPv6 server in a way that some machines will always have static IPv6 address, but rest just random in given range?

There are several mechanisms an ISP may use to provide IPv6 addresses so you need to know which they use. The main issue is that each network requires a /64 prefix, if the ISP provides only one this is fine for a mobile phone as it is the endpoint, however if you are going to route you need one /64 prefix for the 'WAN' connection plus one /64 prefix for each LAN network. See viewtopic.php?t=132247

The European network coordination centre / regional internet registry authored a 'best current operational practice' document https://www.ripe.net/publications/docs/ripe-690 which suggests each customer is provided with a /48 prefix, potentially just a /56 prefix for non-business customers. Similarly the Broadband Forum have a technical report TR-177 https://www.broadband-forum.org/technic ... TR-177.pdf which recommends each customer is provided with a /56 prefix, a /60 as a minimum, and a /48 for larger organisations. Unfortunately some operators ignore these making their IPv6 implementations unusable.