Edit 21.09.2016 Re-opened this question, because of the following observations:
1. No 4G @ home anymore
While LTE worked quite well after the 126.96.36.199 update, a few days after the update I came across a bug (?), which prevents the Jolla C to connect to LTE (800MHz) at home. The first time this happened, my Jolla C showed 4G and some hours later it showed 2G without doing anything.
Nothing worked to get a 4G connection again: Reboot the device, boot without SIM inserted and boot again with SIM inserted, switch from SIM slot 1 to 2 and back, setting network mode from 'Prefer 4G' to any other option and back, disable mobile data connection, restart network connections via Utilities ...
The only thing that worked was to leave the cell (drive to another region). After I came home again, 4G worked correctly.
After that 4G worked well a few days/weeks, but since a week or two, my Jolla C completely refuses to connect 4G again. I tried everything again, but nothing helps.
My GFs Jolla C with the same OS version works just fine with 4G with the same provider. In other regions where 4G didn't work correctly (i.e. at work) before the update, my Jolla C still connects to 4G.
Update 22.09.2016: After fiddling around with the more network modes patch from @coderus (installing, uninstalling, reinstalling ofono package), my Jolla C connected via 4G again @ home yesterday. Let's see how it behaves the next days/weeks...
2. Slow switching from 2G/3G over to 4G / refusing to switch over after reboot
Jolla C is connected via 4G with good reception. After rebooting the phone, Jolla C refuses to connect via 4G and just stays at 2G or 3G. After some time (and maybe cell switching), it connects via 4G.
For testing, I installed the more network modes patch from @coderus. As soon as I select the option "4G" (only), the network switches it's state from "Telekom.de" to "Searching..." and never ever switches back. When I select "4G+3G", the network switches to "Telekom.de" directly without problems, but connects only via 3G.
I need to mention, that I have a very good 4G reception in my work place and at home, so I can clearly say that a "4G dead zone" can not be the problem.
I know the 4G band @ home is 20 (800MHz) and I think @ work it is band 20, too - but I can not clearly say that.
as we came back to Germany from our vacation from Helsinki, I noticed that the Jolla C does not want to connect via LTE in our region. Jolla Phone (Jolla 1) has no problems to connect via LTE here.
I checked the specs and compared them to the Jolla 1 and it seems, that the Jolla C should support all the LTE bands the Jolla 1 supports plus band 1.
Unfortunately I don't know the LTE band used in our region, but I think it is band 20 (800MHz) as we live in a small village.
Is the spec sheet of Jolla C wrong, or is it a bug in Sailfish OS?
Intex itself specifies the LTE bands 2300, 1800 and 850 on their site: http://www.intex.in/mobile/smartphone/aqua-fish/specifications.html
So it seems that your spec sheet is wrong. If this is true it would be a real PITA because 90% of time I only have the 800MHz band and almost never 3G.
Note by @Sailor:
Seems not to be the case: https://twitter.com/JamesNoori/status/755759996659503108
Note by @Armadillo:
Still under investigation: https://twitter.com/awlnx/status/761167055961726982