@BDR45 

Hello,

1. Please confirm with your ISP that the SIM card complies with the Network Type of tp-link mobile wifi device and gets the APN of SIM card. 
Here is the Network Type of M7350 and SIM card should support for example " 4G LTE XX MHz" 

Here is a link you can refer to know the network type of the M7350:

https://www.tp-link.com/en/home-networking/mifi/m7350/#specifications

2. If SIM card supports the network type, please create an APN profile as the instruction below:
https://www.tp-link.com/en/support/faq/794/

Kindly note that the APN information including APN, Username, and password, etc. are provided by your ISP.

3. Please insert SIM into a cell phone and check the Internet.

If still no Internet, please help us confirm some information:
1. When did you buy the tp-link device and when did the issue start?
Does SIM card work fine in your cell phone?

2. May I know who is the ISP?  Did you contact confirm SIM card supports the network type with ISP?

3. After creating the APN file, please wait for 5 mins and help us collect the system log:
Advanced -> System Tools -> System Log-> Save Log and send to us

4.  Advanced -> Dial-up -> Dial-up Settings, make a screenshot  and send to us                                

Advanced -> Dial-up -> ISP Update, check the ISP Version and send to us 
Advanced-> Firmware upgrade -> check the Firmware version and send to us 

5. May I know the hardware version of your Mobile wifi device?

May it help and have a nice day!