Tvheadend

There is an android app called Ping Tools. You should install this on your one+. Then ping the server and make sure that you are getting a response.
If not, you may have a configuration problem.
Your IP address 192.168.50.130 is slightly unusual for a home network in that it has 50 in the 3rd octet, rather than 1. I.e. I would expect it to be 192.168.1.130
If you have a subnet mask of 255.255.255.0 set either device or router (which is often the default), you will not connect because you have a class B IP address defined but a class C mask. You either need to change the mask to 255.255.0.0 OR change the IP of your devices to all begin with 192.168.1.
Lastly if you can ping the server from your phone "Access denied" may refer to your login credentials being entered wrong in the TVH client.

go to your router and see is from there both devices are seen
disable data connection on phone and leave only WiFi

in profile no need web interface and admin
just streaming

@Paraic McDonagh
ip addresses on routers by default variate.
ex.
MikroTik dhcp is 192.168.88.x
some Linksys 10.0.0.x
also when there is "double NAT" second router can't have same network as on WAN port

Most important is addresses to be from private pools

192.168.0.0/16 (255.255.0.0) 192.168.0.0 – 192.168.255.255
172.16.0.0/12  (255.240.0.0) 172.16.0.0 – 172.31.255.255
10.0.0.0/8     (255.0.0.0)   10.0.0.0 – 10.255.255.255

Just to clarify things a little. If you are using DHCP (having your IP addresses assigned automatically), this is a convenient but not very reliable method to configure your network.
You should really assign IP addresses manually for your TVH Server, and client. They should be on the same subnet (First 3 octets the same e.g. "192.168.1.???") as each other and as your router or you will have problems. Having your TVH server connected to the router (and Internet) and ditto for your phone, is not the same thing as being connected to each other via the router.
So to be sure that you don't run into problems, set the first 3 octets to be the same for all devices.
For example: Router = 192.168.1.1, TVH Server = 192.168.1.130, Android Client = 192.168.1.120. Or they can all begin with 192.168.50. but whichever you use, they should all have the same first 3 numbers. If you have DHCP assigning IP addresses but one device is manually set, that can cause issues also.
Do not rely on the error messages from TVH Client. Begin by pinging your router's address from your phone. Next ping the TVH Server. Pinging is the most effective way to check whether IP configuration is the issue.

The error messages you mentioned suggest
1. Network issues. ("Connection timeout").
2. You may have entered the incorrect login details.("Access Denied") when you sometimes do connect.

It goes without saying, your phone needs to be connected to your home router via wi-fi and will not have access if your phone is attempting to communicate over the phone network. But to be sure, disable mobile data as Saen Acro mentioned. Manual IP configuration of your OnePlus is in Settings -> WiFi Then long press/hold the wifi network that represents your home network. This will bring up a context menu, press modify. Then scroll down to where is says IP settings, change it from DHCP to manual. You will then be able to manually enter IP addresses in the fields below.

We could help more if you told us more about the IP addresses set on those 3 devices and a bit more about your router.

Hi, first of all thankx to all for helping me.
I´m using the private IP range 192.168.50.* for all devices in my home network.
"Ping Tools" is a great app, I didn´t know before. From the Android device I´m able to ping 192.168.50.130 (the Ubuntu maschine which is running tvheadend), but there is no answer from 192.168.50.130:9981 or 192.168.50.130:9982 which I imagine is correct.

After a reboot of the Android device I could manage to connect with TVHGuide to tvheadend, however TVHClient is reporting the same error as described in my first post using the same login data as I used with TVHGuide. So I can imagine, that it is a problem with TVHClient and my Oneplus because I´m using the device without Google Playstore.

TVHGuide is showing me all TV channels and its EPG and also recordings. What is not working is streaming but even here I think I have to configure something by myself because there is no Playstore for standard applications.

So some question please:
Playback port is 9981 or 9982?
I´m trying with VLC, but it is not working. There is a error message popping up, I can´t read. Will install a screen record app and try to debug.

Also what´s not working: If I mark a show to record at TVHGuide it will not be added to the records list, not at TVHGuide and not at tvheadend.

Will continue reporing after I looked into the log files.