Does SIM-less tablet device supports internet modem? I have SAMSUNG GALAXY TAB 3 (GT-P5210). It does not have a SIM slot and so I can't call to anyone without using VoIP software and Internet data. Does this device support external modem (eg: Huawei)?
I'm just trying to understand what your question is and give you an answer to it... 

The GT-P5210 has the Wi-Fi module so it can connect to devices that create a wireless network. If the "external internet modem" you're talking about has the capability to create a wireless network then you're good to go. 
If your question was something like "can I make phone call with my wifi-only tablet?" then the answer is: theoretically no. And when I say THEORETICALLY I mean that the tablet itself cannot make phone calls but there are various way to sort the thing out (voip apps just to make an example)...