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.