Hello Mihau,
Welcome to the community!
Firstly, since you purchased the phone from China, are you sure the model number is D5803? That version of the phone is not very compatible with the networks over there (see here). China has its own versions of the Xperia Z3, namely L55t and L55u (see here). If you have one of the Chinese variants, it may or may not have compatible bands/frequencies.
That said, assuming the version you have is the D5803, these are its LTE bands/frequencies:
1(2100), 2(1900), 3(1800), 4(1700/2100), 5(850), 7(2600), 8(900), 13(700), 17(700), 20(800) ~ taken from here.
According to those specifications, that version of the phone should have three of the compatible bands/frequencies. You can verify the compatible bands/frequencies here.
To clarify, since the phone is LTE capable, it should be using ltemobile.apn You can verify the correct APN settings here.
Have you verified your data is enabled? Sending and/or receiving MMS requires data to be enabled.
Hope this helps 
Cheers
Hello again,
Thanks for the additional information!
@mihau wrote:
...
Wnen I check configuation with the same app, it shows the following working bands: GSM_850, GSM _900, GSM_1800, GSM_1900,UMTS_BC1, UMTS_BC5, UMTSBC6, UMTS_BC6, UMTS_BC6,UMTS_BC19,
LTE_BC3, LTE_BC3, LTE_BC19, LTE_BC21. (All that is rather confusing to me... )...
Are those all of the bands available on your phone? Also, do you mind verifying the UMTS and LTE bands? You have listed UMTS_BC6 a couple of times and LTE_BC3 twice...
According to those bands, that phone has all of the compatible bands for 2G (GSM). On the other hand, that phone would only have one band for '3G' (UMTS). Unfortunately, those bands do not list any compatible LTE bands.
I understand that the phone lists the model number as D5803. Did you purchase the phone from a reputable source? Is it possible that the phone might not be genuine?
It's also possible that the phone is region locked to Hong Kong (see here). Note that this is different from a network lock so it could still be in place even though your phone is technically unlocked. If this is the case, I'm not sure how to region unlock the device.
@mihau wrote:
...
...
When I check for Service providers in Mobile Network Settings on the phone, I get option of 3G Fido, 2G Fido,
3G Fido-EXT, (this one shows twice), or 3G 302520, however only 3G Fido option works. There is no LTE option...
Just for your information, Fido-EXT refers to extended coverage. With the roaming agreements, Fido customers can roam using the other cellular towers (Bell and Telus) as Fido-EXT when outside of Fido/Rogers coverage. If your phone has Fido/Rogers coverage, then it won't allow you onto Fido-EXT. The 3G 302520 would refer to Videotron's tower.
mihau wrote:...
But even if the phone were not compatable with Fido's LTE bands, (which I can live with for a while,
until I get another one), I still should get Fido service in Montreal metro, like Lionel
Berri_UQAM station, for instance, right? But I dont... Any ideas?
Cellular reception can be affected by many factors, including building interference. Compounding the issue, however, is your phone's available bands/frequencies.
Taken from here.
The closest tower to the station appears to only have 3G 1900 MHz (band 2). Unfortunately, your phone does not have that band available.
With regards to the mobile network within the transit system, you might consider trying to connect to Fido-EXT since its deployment was a joint venture.
Hope this helps
Cheers
Hello,
Sorry, writing UMTS_BC6 three times(!) and LTE_BC3 twice were typos, got distracted, I guess..
Apart of this mistake, I mentioned all working bands that the Xperia app is showing.
Is it really D5803? Reputable source?....Well, I do not know, can't really tell. My loss, I guess.
I will have to live with the 3G for a while. (I'm starting to look for another phone already.)
Regarding the Fido-EXT, would I have to switch manualy to EXT each time in metro?
When I try to switch to Fido-EXT while at home, a message says that this provider is not available.
I remember, while traveling in Quebec, that my old phone would connect to EXT itself,
I did not have to do anything. Isn't it the case in the transit system? It's not a big deal but there are days
when I spend like an hour in the metro, so having internet connection, without a hasle of switching it
each time, would not hurt. Anyways, I'll try it next time I have a chance.
Anyways, I really appreciate your help and, as I said earlier, a new phone will be on its way soon:)
Hello again,
Thanks for verifying that information! As previously suspected, that phone only has one available '3G' band and no LTE bands. Coverage on the '3G' network would depend on corresponding bands on your surrounding towers.
You're correct, normally the phones should automatically switch to Fido-EXT when outside of Fido/Rogers coverage. Occasionally, if on the very edge of coverage, the phone might get intermittent signal preventing the phone from automatically switching to extended coverage. In those cases, manually switching networks may be possible.
Hope you don't have to wait too long for your new phone!
Cheers
