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Those symbols indicate which cellular network standard your mobile data connection is currently using, which in turn determines your maximum connection speed.
H stands for HSPA, or High Speed Packet Access, with a maximum theoretical speed of 14 Mb/s, or 42 Mb/s with HSDPA (High Speed Downlink Packet Access).
H+ stands for HSPA+, or Evolved HSPA, with a ...
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It may just be that the APN settings have got lost.
If you know what they are for your network, you can go to settings > cellular (assuming US English) and edit the APN in there
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Before turning on internet sharing, you should enable your cellular data connection
http://www.windowsphone.com/en-us/how-to/wp7/start/cellular-settings
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